tui: dump termcap info if -V3 ('verbose' >= 3)

Get terminal debugging info by starting Nvim with 'verbose' level 3:

    nvim -V3log

This is like Vim's `:set termcap`, which was removed in Nvim (and would
be very awkward to restore because of the decoupled UI).

runtime/doc/eval.txt

@@ -8889,11 +8889,6 @@ This does NOT work: >
 			value and the global value are changed.
 			Example: >
 				:let &path = &path . ',/usr/local/include'
-<			This also works for terminal codes in the form t_xx.
-			But only for alphanumerical names.  Example: >
-				:let &t_k1 = "\<Esc>[234;"
-<			When the code does not exist yet it will be created as
-			a terminal key code, there is no error.
 
 :let &{option-name} .= {expr1}
 			For a string option: Append {expr1} to the value.

runtime/doc/options.txt

@@ -6424,6 +6424,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	Currently, these messages are given:
 	>= 1	When the shada file is read or written.
 	>= 2	When a file is ":source"'ed.
+	>= 3	UI info, terminal capabilities
 	>= 5	Every searched tags file and include file.
 	>= 8	Files for which a group of autocommands is executed.
 	>= 9	Every executed autocommand.

runtime/doc/starting.txt

@@ -249,14 +249,14 @@ argument.
 		for reading or writing a ShaDa file.  Can be used to find
 		out what is happening upon startup and exit.
 		Example: >
-			vim -V8 foobar
+			nvim -V8
 
 -V[N]{filename}
-		Like -V and set 'verbosefile' to {filename}.  The result is
-		that messages are not displayed but written to the file
-		{filename}.  {filename} must not start with a digit.
+		Like -V and set 'verbosefile' to {filename}.  Messages are not
+		displayed; instead they are written to the file {filename}.
+		{filename} must not start with a digit.
 		Example: >
-			vim -V20vimlog foobar
+			nvim -V20vimlog
 <
 							*-D*
 -D		Debugging.  Go to debugging mode when executing the first

runtime/doc/vim_diff.txt

@@ -327,22 +327,26 @@ Ed-compatible mode:
   ":set noedcompatible" is ignored
   ":set edcompatible" is an error
 
-	*t_xx* *:set-termcap* *termcap-options* *t_AB* *t_Sb* *t_vb* *t_SI*
+			*t_xx* *termcap-options* *t_AB* *t_Sb* *t_vb* *t_SI*
 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*
+			*:set-termcap*
+Start Nvim with 'verbose' level 3 to see the terminal capabilities. >
+	nvim -V3
+<
+			*'term'* *E529* *E530* *E531*
 'term' reflects the terminal type derived from |$TERM| and other environment
 checks.  For debugging only; not reliable during startup. >
 	:echo &term
 "builtin_x" means one of the |builtin-terms| was chosen, because the expected
 terminfo file was not found on the system.
 
-								*termcap*
+			*termcap*
 Nvim never uses the termcap database, only |terminfo| and |builtin-terms|.
 
-						*xterm-8bit* *xterm-8-bit*
+			*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
 mode, as the 8-bit CSI character has to be written differently in each case.