exports.5: Document correct behaviour for -alldirs

[rmacklem]

Summary

The Example section of exports.5 described that a
directory exported with -alldirs would fail if the
directory was not a server file system mount point.

In fact, this behaviour has not been the case since
FreeBSD2 was released.

This patch updates exports(5) for the behaviour
of -alldirs with/without mountd being started with
the "-a" option described in D48137.

The current verbiage does need improvement.

[markj]

The current verbiage does need improvement.

Maybe an introduction which explains FSIDs and file handles, and more generally the relationship between NFS exports and local mount points, would make it easier to simplify some of the rest of the text. I suspect a lot of new readers don't have that background and think that NFS is just a way to export arbitrary directory hierarchies over a network, like 9p.