doc: Document some more terminal behaviours.

This documents 256-colour and true colour handling, cursor shapes,
and scrolling regions.

Almost all of these headings are taken from the Vim doco, so that
the :help commands that people learn are a transferable skill.

runtime/doc/vim_diff.txt

@@ -156,6 +156,10 @@ are always available and may be used simultaneously in separate plugins.  The
 `neovim` pip package must be installed to use Python plugins in Nvim (see
 |provider-python|).
 
+Because of general |256-color| usage whereever possible, Nvim will even use
+256-colour capability on Linux virtual terminals.  Vim uses only 8 colours
+plus bright foreground on Linux VTs.
+
 |:!| does not support "interactive" commands. Use |:terminal| instead.
 (GUI Vim has a similar limitation, see ":help gui-pty" in Vim.)
 
@@ -281,6 +285,11 @@ Nvim does not have special `t_XX` options nor <t_XX> keycodes to configure
 terminal capabilities. Instead Nvim treats the terminal as any other UI. For
 example, 'guicursor' sets the terminal cursor style if possible.
 
+								*termcap*
+Nvim never uses the termcap database and only uses |terminfo|.  See
+|builtin-terms| for what happens on operating systems without a terminfo
+database.
+
 						*xterm-8bit* *xterm-8-bit*
 Xterm can be run in a mode where it uses true 8-bit CSI.  Supporting this
 requires autodetection of whether the terminal is in UTF-8 mode or non-UTF-8