debugfs: document that debugfs_remove*() accepts NULL and error values

According to commit a59d6293e537 ("debugfs: change parameter check in
debugfs_remove() functions"), this is meant to make cleanup easier for
callers. In that case it ought to be documented.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

fs/debugfs/inode.c

@@ -533,7 +533,8 @@ static int __debugfs_remove(struct dentry *dentry, struct dentry *parent)
 /**
  * debugfs_remove - removes a file or directory from the debugfs filesystem
  * @dentry: a pointer to a the dentry of the file or directory to be
- *          removed.
+ *          removed.  If this parameter is NULL or an error value, nothing
+ *          will be done.
  *
  * This function removes a file or directory in debugfs that was previously
  * created with a call to another debugfs function (like
@@ -565,7 +566,8 @@ EXPORT_SYMBOL_GPL(debugfs_remove);
 
 /**
  * debugfs_remove_recursive - recursively removes a directory
- * @dentry: a pointer to a the dentry of the directory to be removed.
+ * @dentry: a pointer to a the dentry of the directory to be removed.  If this
+ *          parameter is NULL or an error value, nothing will be done.
  *
  * This function recursively removes a directory tree in debugfs that
  * was previously created with a call to another debugfs function