docs: tmpfs: Add casefold options

Document mounting options for casefold support in tmpfs.

Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/r/20241021-tonyk-tmpfs-v8-9-f443d5814194@igalia.com


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