libmount: update documentation for MNT_ERR_APPLYFLAGS

libmount: update documentation for MNT_ERR_APPLYFLAGS

The implementation using the new FD based mount kernel API (ie.,
fsconfig/fsopen) uses MNT_ERR_APPLYFLAGS for failed mount_setattr(2)
calls, which involves more mount attributes (eg., MOUNT_ATTR_RDONLY,
MOUNT_ATTR_NOSUID, etc.) in addition to the MS_PROPAGATION flags (eg.,
MS_SHARED, MS_UNBINDABLE, etc.).

Note that mount_setattr(2) is part of the new FD based mount kernel API,
and is not used by the classic mount(2) based version.

Fallout from 987d844cdbc0f91ca81de3c1e5d0628a60eb458f

Signed-off-by: Debarshi Ray <rishi@fedoraproject.org>