fetch-pack: document only_packfile in get_pack()

dd4b732df7 ("upload-pack: send part of packfile response as uri",
2020-06-10) added the "only_packfile" parameter to get_pack() but did
not document it. Add documentation.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fetch-pack.c

@@ -794,6 +794,10 @@ static void write_promisor_file(const char *keep_name,
 	strbuf_release(&promisor_name);
 }
 
+/*
+ * Pass 1 as "only_packfile" if the pack received is the only pack in this
+ * fetch request (that is, if there were no packfile URIs provided).
+ */
 static int get_pack(struct fetch_pack_args *args,
 		    int xd[2], struct string_list *pack_lockfiles,
 		    int only_packfile,