exports(5): Revert warning about `-ro' on read/write file systems.

Looks like the nfs server does enforce that after all, in spite of
the rather oblique commentary in the BUGS section about export
options being tied to local mount point options with which they must
be noncontradictory.

And there's no reason in principle it shouldn't enforce this -- it
just need to block various file system _operations_, rather than the
subtree issue where the criteria for evaluating whether operations
are allowed on particular _file handles_ are too painful to
contemplate.

PR misc/58063

(riastradh)