doco: Document constant 'term' in more detail.

Also expand the example in term-dependent-settings.

runtime/doc/term.txt

@@ -90,12 +90,22 @@ Settings depending on terminal			*term-dependent-settings*
 If you want to set options or mappings, depending on the terminal name, you
 can do this best in your init.vim.  Example: >
 
-    if $TERM =~ '^\(rxvt\|screen\)\(\|-.*\)'
+    if $TERM =~ '^\(rxvt\|screen\|interix\|putty\)\(-.*\)\?$'
         set notermguicolors
-    elseif $TERM =~ '^\(xterm\|tmux\)\(\|-.*\)'
+    elseif $TERM =~ '^\(tmux\|iterm\|vte\|gnome\)\(-.*\)\?$'
         set termguicolors
+    elseif $TERM =~ '^\(xterm\)\(-.*\)\?$'
+        if $XTERM_VERSION != ''
+            set termguicolors
+        elseif $KONSOLE_PROFILE_NAME != ''
+            set termguicolors
+        elseif $VTE_VERSION != ''
+            set termguicolors
+        else
+            set notermguicolors
+        endif
     elseif $TERM =~ ...
-	... and so forth ...
+        ... and so forth ...
     endif
 <
 					*scroll-region* *xterm-scroll-region*

runtime/doc/vim_diff.txt

@@ -292,6 +292,12 @@ 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.
 
+						  *'term'* *E529* *E530* *E531*
+The 'term' option has a fixed value, present only for script compatibility and
+intentionally not the same as any known terminal type name.  It should be a
+rare case in Nvim where one needs |term-dependent-settings|, for which use the
+|TERM| environment variable.
+
 								*termcap*
 Nvim never uses the termcap database and only uses |terminfo|.  See
 |builtin-terms| for what happens on operating systems without a terminfo
@@ -346,7 +352,6 @@ Other options:
   'shelltype'
   *'shortname'* *'sn'* *'noshortname'* *'nosn'*
   *'swapsync'* *'sws'*
-  *'term'* *E529* *E530* *E531*
   *'termencoding'* *'tenc'* (Vim 7.4.852 also removed this for Windows)
   'textauto'
   'textmode'