shallow.h: document '{commit,rollback}_shallow_file'

When 'commit_shallow_file()' and 'rollback_shallow_file()' were
introduced, they did not have a documenting comment, when they could
have benefited from one.

Add a brief note about what these functions do, and make a special note
that they reset stat-validity checks.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

shallow.h

@@ -10,7 +10,9 @@ void set_alternate_shallow_file(struct repository *r, const char *path, int over
 int register_shallow(struct repository *r, const struct object_id *oid);
 int unregister_shallow(const struct object_id *oid);
 int is_repository_shallow(struct repository *r);
+/* commit $GIT_DIR/shallow and reset stat-validity checks */
 int commit_shallow_file(struct repository *r, struct lock_file *lk);
+/* rollback $GIT_DIR/shallow and reset stat-validity checks */
 void rollback_shallow_file(struct repository *r, struct lock_file *lk);
 
 struct commit_list *get_shallow_commits(struct object_array *heads,