docs: Remove shortname references.

In addition, align the removal notice next to 'key' with other
such notices (options.txt).

runtime/doc/editing.txt

@@ -136,22 +136,8 @@ as the original file with 'backupext' appended.  The default "~" is a bit
 strange to avoid accidentally overwriting existing files.  If you prefer ".bak"
 change the 'backupext' option.  Extra dots are replaced with '_' on MS-DOS
 machines, when Vim has detected that an MS-DOS-like filesystem is being used
-(e.g., messydos or crossdos) or when the 'shortname' option is on.  The
-backup file can be placed in another directory by setting 'backupdir'.
-
-							*auto-shortname*
-Technical: On the Amiga you can use 30 characters for a file name.  But on an
-	   MS-DOS-compatible filesystem only 8 plus 3 characters are
-	   available.  Vim tries to detect the type of filesystem when it is
-	   creating the .swp file.  If an MS-DOS-like filesystem is suspected,
-	   a flag is set that has the same effect as setting the 'shortname'
-	   option.  This flag will be reset as soon as you start editing a
-	   new file.  The flag will be used when making the file name for the
-	   ".swp" and ".~" files for the current file.  But when you are
-	   editing a file in a normal filesystem and write to an MS-DOS-like
-	   filesystem the flag will not have been set.  In that case the
-	   creation of the ".~" file may fail and you will get an error
-	   message.  Use the 'shortname' option in this case.
+(e.g., messydos or crossdos).  The backup file can be placed in another
+directory by setting 'backupdir'.
 
 When you started editing without giving a file name, "No File" is displayed in
 messages.  If the ":write" command is used with a file name argument, the file

runtime/doc/eval.txt

@@ -2811,7 +2811,6 @@ exists({expr})	The result is a Number, which is non-zero if {expr} is
 		For checking for a supported feature use |has()|.
 
 		Examples: >
-			exists("&shortname")
 			exists("$HOSTNAME")
 			exists("*strftime")
 			exists("*s:MyFunc")

runtime/doc/options.txt

@@ -4390,7 +4390,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	NOTE: This option is set when 'compatible' is set.
 
 					*'key'*
-'key'	Removed. {Nvim}
+'key'			Removed. {Nvim}
 
 					*'keymap'* *'kmp'* *E544*
 'keymap' 'kmp'		string	(default "")
@@ -6209,17 +6209,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	set and to the Vim default value when 'compatible' is reset.
 
 				 *'shortname'* *'sn'* *'noshortname'* *'nosn'*
-'shortname' 'sn'	boolean	(default off)
-			local to buffer
-			{not in Vi, not in MS-DOS versions}
-	Filenames are assumed to be 8 characters plus one extension of 3
-	characters.  Multiple dots in file names are not allowed.  When this
-	option is on, dots in file names are replaced with underscores when
-	adding an extension (".~" or ".swp").  This option is not available
-	for MS-DOS, because then it would always be on.  This option is useful
-	when editing files on an MS-DOS compatible filesystem, e.g., messydos
-	or crossdos.  When running the Win32 GUI version under Win32s, this
-	option is always on by default.
+'shortname' 'sn'	Removed. {Nvim}
 
 						*'showbreak'* *'sbr'* *E595*
 'showbreak' 'sbr'	string	(default "")

runtime/doc/quickref.txt

@@ -857,7 +857,6 @@ Short explanation of each option:		*option-list*
 'shiftround'	  'sr'	    round indent to multiple of shiftwidth
 'shiftwidth'	  'sw'	    number of spaces to use for (auto)indent step
 'shortmess'	  'shm'     list of flags, reduce length of messages
-'shortname'	  'sn'	    non-MS-DOS: Filenames assumed to be 8.3 chars
 'showbreak'	  'sbr'     string to use at the start of wrapped lines
 'showcmd'	  'sc'	    show (partial) command in status line
 'showfulltag'	  'sft'     show full tag pattern when completing tag

runtime/doc/recover.txt

@@ -33,20 +33,13 @@ with the extension ".swp".
 - On Unix, a '.' is prepended to swap file names in the same directory as the
   edited file.  This avoids that the swap file shows up in a directory
   listing.
-- On MS-DOS machines and when the 'shortname' option is on, any '.' in the
-  original file name is replaced with '_'.
 - If this file already exists (e.g., when you are recovering from a crash) a
   warning is given and another extension is used, ".swo", ".swn", etc.
 - An existing file will never be overwritten.
 - The swap file is deleted as soon as Vim stops editing the file.
 
-Technical: The replacement of '.' with '_' is done to avoid problems with
-	   MS-DOS compatible filesystems (e.g., crossdos, multidos).  If Vim
-	   is able to detect that the file is on an MS-DOS-like filesystem, a
-	   flag is set that has the same effect as the 'shortname' option.
-	   This flag is reset when you start editing another file.
 							*E326*
-	   If the ".swp" file name already exists, the last character is
+Technical: If the ".swp" file name already exists, the last character is
 	   decremented until there is no file with that name or ".saa" is
 	   reached.  In the last case, no swap file is created.
 

runtime/doc/todo.txt

@@ -4817,8 +4817,7 @@ Digraphs:
 
 Writing files:
 -   In vim_rename(), should lock "from" file when deleting "to" file for
-    systems other than Amiga.  Avoids problems with unexpected longname to
-    shortname conversion.
+    systems other than Amiga.
 8   write mch_isdevice() for Amiga, Mac, VMS, etc.
 8   When appending to a file, Vim should also make a backup and a 'patchmode'
     file.

runtime/doc/vi_diff.txt

@@ -751,10 +751,6 @@ switched off by setting the 'updatecount' option to 0 or starting Vim with
 the "-n" option.  Use the 'directory' option for placing the .swp file
 somewhere else.
 
-Vim is able to work correctly on filesystems with 8.3 file names, also when
-using messydos or crossdos filesystems on the Amiga, or any 8.3 mounted
-filesystem under Unix.  See |'shortname'|.
-
 Error messages are shown at least one second (Vi overwrites error messages).
 
 If Vim gives the |hit-enter| prompt, you can hit any key.  Characters other