doc: "terminal" always means "embedded terminal emulator"

- Prefer "TUI" where possible to refer to the host terminal.
- Remove obsolete tags and ancient TTY exposition.
- Establish "terminal" to consistently mean "terminal emulator" in all
  Nvim documentation. This removes the need for verbose qualifiers in
  tags and prose.

References #6280
References #6803

scripts/vim-patch.sh

@@ -135,7 +135,7 @@ preprocess_patch() {
   local na_src='proto\|Make*\|gui_*\|if_lua\|if_mzsch\|if_olepp\|if_ole\|if_perl\|if_py\|if_ruby\|if_tcl\|if_xcmdsrv'
   2>/dev/null $nvim --cmd 'set dir=/tmp' +'g@^diff --git a/src/\S*\<\%('${na_src}'\)@norm! d/\v(^diff)|%$
 ' +w +q "$file"
-
+
   # Remove channel.txt, netbeans.txt, os_*.txt, term.txt, todo.txt, version*.txt, tags
   local na_doc='channel\.txt\|netbeans\.txt\|os_\w\+\.txt\|term\.txt\|todo\.txt\|version\d\.txt\|tags'
   2>/dev/null $nvim --cmd 'set dir=/tmp' +'g@^diff --git a/runtime/doc/\<\%('${na_doc}'\)\>@norm! d/\v(^diff)|%$