exports.5: Document the current behavior of -alldirs

Commit 07cd69e272da adds a new "-a" mountd option,
which changes the behavior of mountd when file systems
are exported via -alldirs.

This patch updates the man page to reflect the actual
behavior when -alldirs is used when mountd is started
with/without -a. Prior to the above commit, exports(5)
documented that, when -alldirs was specified, the exports
line would fail unless the directory was a server file
system mount point. This behavior was only documented
in the Examples section and has not been implemented
since a change between FreeBSD 1 and FreeBSD 2 was done.

This is a contents change.

PR: 282995
Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D48139