patch 9.1.1340: cannot complete :filetype arguments

Problem:  cannot complete :filetype arguments (Phạm Bình An)
Solution: add :filetype ex command completion, add "filetypecmd"
          completion type for getcompletion()

fixes: #17165
closes: #17167

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 0a1c3b1..e6747ad 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -4270,6 +4270,7 @@
 		file		file and directory names
 		file_in_path	file and directory names in |'path'|
 		filetype	filetype names |'filetype'|
+		filetypecmd	|:filetype| suboptions
 		function	function name
 		help		help subjects
 		highlight	highlight groups