exportfs: clarify the documentation of open()/permission() expotrfs ops

exportfs: clarify the documentation of open()/permission() expotrfs ops

pidfs and nsfs recently gained support for encode/decode of file handles
via name_to_handle_at(2)/open_by_handle_at(2).

These special kernel filesystems have custom ->open() and ->permission()
export methods, which nfsd does not respect and it was never meant to be
used for exporting those filesystems by nfsd.

Update kernel-doc comments to express the fact the those methods are for
open_by_handle(2) system only and not compatible with nfsd.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://patch.msgid.link/20260129100212.49727-2-amir73il@gmail.com
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>