docs: Document the FAN_FS_ERROR event

Document the FAN_FS_ERROR event for user administrators and user space
developers.

Link: https://lore.kernel.org/r/20211025192746.66445-32-krisman@collabora.com


Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jan Kara <jack@suse.cz>