completion: improve __git_refs()'s in-code documentation

That "first argument is passed to __gitdir()" statement in particular
is not really helpful, and after this series it won't be the case
anyway.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -332,9 +332,11 @@ __git_tags ()
 	fi
 }
 
-# __git_refs accepts 0, 1 (to pass to __gitdir), or 2 arguments
-# presence of 2nd argument means use the guess heuristic employed
-# by checkout for tracking branches
+# Lists refs from the local (by default) or from a remote repository.
+# It accepts 0, 1 or 2 arguments:
+# 1: The remote to list refs from (optional; ignored, if set but empty).
+# 2: In addition to local refs, list unique branches from refs/remotes/ for
+#    'git checkout's tracking DWIMery (optional; ignored, if set but empty).
 __git_refs ()
 {
 	local i hash dir="$(__gitdir "${1-}")" track="${2-}"