encoding: update docs for encoding

Helped-By: Michael Reed <m.reed@mykolab.com>
Helped-By: Justin M. Keyes <justinkz@gmail.com>

runtime/doc/mbyte.txt

@@ -89,8 +89,8 @@ See |encoding-values| for a list of acceptable values.
 
 The result is that all the text that is used inside Vim will be in this
 encoding.  Not only the text in the buffers, but also in registers, variables,
-etc.  This also means that changing the value of 'encoding' makes the existing
+etc. 'encoding' cannot be changed after startup (at latest when |vimrc|
-text invalid!  The text doesn't change, but it will be displayed wrong.
+is sourced) as changing the value later would make the existing text invalid.
 
 You can edit files in another encoding than what 'encoding' is set to.  Vim
 will convert the file when you read it and convert it back when you write it.

runtime/doc/message.txt

@@ -739,6 +739,12 @@ Example: >
 You tried to execute a command that is neither an Ex command nor
 a user-defined command.
 
+							*E905*  >
+  Cannot set this option after startup
+
+This option might only be changed in a |vimrc| file (or earlier). This means
+that plugins can rely on the value being constant after initialization.
+
 ==============================================================================
 3. Messages						*messages*
 

runtime/doc/options.txt

@@ -2217,21 +2217,13 @@ A jump table for the options with a short description can be found at |Q_op|.
 	viminfo file, etc.  It sets the kind of characters which Vim can work
 	with.  See |encoding-names| for the possible values.
 
-	NOTE: Changing this option will not change the encoding of the
+	This option cannot be changed after startup.
-	existing text in Vim.  It may cause non-ASCII text to become invalid.
+	Otherwise it would cause non-ASCII text inside Vim to become
-	It should normally be kept at its default value, or set when Vim
+	invalid. It should normally be kept at its default value, or be set
-	starts up.  See |multibyte|.  To reload the menus see |:menutrans|.
+	in vimrc. See |multibyte|.
 
-	This option cannot be set from a |modeline|.  It would most likely
+	The recommended 'encoding' is "utf-8". Remote plugins and GUI:s
-	corrupt the text.
+	only support utf-8.
-
-	NOTE: For GTK+ 2 it is highly recommended to set 'encoding' to
-	"utf-8".  Although care has been taken to allow different values of
-	'encoding', "utf-8" is the natural choice for the environment and
-	avoids unnecessary conversion overhead.  "utf-8" has not been made
-	the default to prevent different behavior of the GUI and terminal
-	versions, and to avoid changing the encoding of newly created files
-	without your knowledge (in case 'fileencodings' is empty).
 
 	The character encoding of files can be different from 'encoding'.
 	This is specified with 'fileencoding'.  The conversion is done with
@@ -2243,6 +2235,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 <
 	Normally 'encoding' will be equal to your current locale.  This will
 	be the default if Vim recognizes your environment settings.
+	"utf-8" is used when the locale encoding could not be detected.
 
 	When you set this option, it fires the |EncodingChanged| autocommand
 	event so that you can set up fonts if necessary.
@@ -2254,12 +2247,6 @@ A jump table for the options with a short description can be found at |Q_op|.
 	For example "Latin-1" becomes "latin1", "ISO_88592" becomes
 	"iso-8859-2" and "utf8" becomes "utf-8".
 
-	Note: "latin1" is also used when the encoding could not be detected.
-	This only works when editing files in the same encoding!  When the
-	actual character set is not latin1, make sure 'fileencoding' and
-	'fileencodings' are empty.  When conversion is needed, switch to using
-	utf-8.
-
 	When "unicode", "ucs-2" or "ucs-4" is used, Vim internally uses utf-8.
 	You don't notice this while editing, but it does matter for the
 	|viminfo-file|.  And Vim expects the terminal to use utf-8 too.  Thus

runtime/doc/vim_diff.txt

@@ -74,6 +74,8 @@ are always available and may be used simultaneously in separate plugins.  The
    'p')) mkdir() will silently exit. In Vim this was an error.
 3. mkdir() error messages now include strerror() text when mkdir fails.
 
+'encoding' cannot be changed after startup.
+
 ==============================================================================
 4. New Features						     *nvim-features-new*