feat(complete): support f flag for complete buffer part

runtime/doc/news.txt

@@ -195,6 +195,8 @@ The following new APIs and features were added.
 
 • Added |vim.snippet| for snippet expansion support.
 
+• 'complete' option supports "f" flag for completing buffer names.
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changed*
 

runtime/doc/options.txt

@@ -1458,6 +1458,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 		|i_CTRL-X_CTRL-D|
 	]	tag completion
 	t	same as "]"
+	f	scan the buffer names (as opposed to buffer contents)
 
 	Unloaded buffers are not loaded, thus their autocmds |:autocmd| are
 	not executed, this may lead to unexpected completions from some files