vim-patch:9.2.1004: a completion function cannot tell why it was called (#41500)

Problem:  A function used through 'omnifunc' or 'complete' is called the
          same way whether 'autocomplete' started the completion or a
          key asked for one, so it cannot answer differently.
Solution: Report which of the two it is in complete_info() as "auto". It
          is returned only when asked for in {what}, so what
          complete_info() says by itself does not change
          (Hirohito Higashi).

closes: vim/vim#21143

https://github.com/vim/vim/commit/1f56c351dedad288f79c89b6b6f64be762ec3b9b

Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>

runtime/doc/vimfn.txt

@@ -1316,6 +1316,12 @@ complete_info([{what}])                                        *complete_info()*
 		Returns a |Dictionary| with information about Insert mode
 		completion.  See |ins-completion|.
 		The items are:
+		   auto		|TRUE| when Vim started this completion by
+				itself, which is what 'autocomplete' does,
+				and |FALSE| when a key asked for it, such as
+				|i_CTRL-X_CTRL-O|.  A |complete-functions|
+				function can read this to tell the two apart.
+				Returned only when asked for in {what}.
 		   completed	Return a dictionary containing the entries of
 				the currently selected index item.
 		   items	List of all completion candidates.  Each item