fs: add kernel doc for mnt_{hold,unhold}_writers()

When I introduced mnt_{hold,unhold}_writers() in commit fbdc2f6
("fs: split out functions to hold writers") I did not add kernel doc for
them. Fix this and introduce proper documentation.

Link: https://lore.kernel.org/r/20220203131411.3093040-4-brauner@kernel.org
Fixes: fbdc2f6 ("fs: split out functions to hold writers")
Cc: Seth Forshee <seth.forshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -469,6 +469,24 @@ void mnt_drop_write_file(struct file *file)
 }
 EXPORT_SYMBOL(mnt_drop_write_file);
 
+/**
+ * mnt_hold_writers - prevent write access to the given mount
+ * @mnt: mnt to prevent write access to
+ *
+ * Prevents write access to @mnt if there are no active writers for @mnt.
+ * This function needs to be called and return successfully before changing
+ * properties of @mnt that need to remain stable for callers with write access
+ * to @mnt.
+ *
+ * After this functions has been called successfully callers must pair it with
+ * a call to mnt_unhold_writers() in order to stop preventing write access to
+ * @mnt.
+ *
+ * Context: This function expects lock_mount_hash() to be held serializing
+ *          setting MNT_WRITE_HOLD.
+ * Return: On success 0 is returned.
+ *	   On error, -EBUSY is returned.
+ */
 static inline int mnt_hold_writers(struct mount *mnt)
 {
 	mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
@@ -500,6 +518,18 @@ static inline int mnt_hold_writers(struct mount *mnt)
 	return 0;
 }
 
+/**
+ * mnt_unhold_writers - stop preventing write access to the given mount
+ * @mnt: mnt to stop preventing write access to
+ *
+ * Stop preventing write access to @mnt allowing callers to gain write access
+ * to @mnt again.
+ *
+ * This function can only be called after a successful call to
+ * mnt_hold_writers().
+ *
+ * Context: This function expects lock_mount_hash() to be held.
+ */
 static inline void mnt_unhold_writers(struct mount *mnt)
 {
 	/*