docs/vfs: Remove mentions to the old mount API helpers

Now that mount_bdev(), mount_nodev() and mount_single() have all been
removed, remove mentions to them in vfs.rst.

While we're at it, redirect people looking for mount API docs to
mount_api.rst (which documents the newer API).

Signed-off-by: Pedro Falcato <pfalcato@suse.de>
Link: https://lore.kernel.org/20250723132156.225410-4-pfalcato@suse.de


Signed-off-by: Christian Brauner <brauner@kernel.org>