Clarify how exports work for administrative controls

[rmacklem]

Summary

There appears to be some confusion w.r.t. how exports
are applied to "administrative controls", which are exported
subdirectories within a server's local file system.

This patch attempts to clarify that the exports apply to the
entire server local file system and not the subtree below the
exported directory.

[emaste]

makes sense to me. a couple of little notes on phrasing

usr.sbin/mountd/exports.5

626

this seems to imply the server has only one local file system, although I'm not sure what would be best

"any directory within a local file system on a server"
"any directory within any of a server's local file systems"

627

maybe "is exported, the entire file system is exported and not just the subtree..."
i.e. "those exports" doesn't seem to be needed

[rmacklem]

Made the changes suggested by emaste@.

[gbe]

LGTM

[emaste]

See also: D44502

[markj]

I'd somewhat prefer to make this more prominent, having its own paragraph in the description section. I understand that the problem at hand is a natural consequence of how exports(5) works, but it's really easy for someone new to NFS to miss that they can't "safely" export a single subdirectory from a local filesystem on the server.

[rmacklem]

In D44499#1015512, @markj wrote:

I'd somewhat prefer to make this more prominent, having its own paragraph in the description section. I understand that the problem at hand is a natural consequence of how exports(5) works, but it's really easy for someone new to NFS to miss that they can't "safely" export a single subdirectory from a local filesystem on the server.

Go for it. I won't commit this one and will be happy to review
whatever you come up with.

[markj]

In D44499#1015675, @rmacklem wrote:

In D44499#1015512, @markj wrote:

I'd somewhat prefer to make this more prominent, having its own paragraph in the description section. I understand that the problem at hand is a natural consequence of how exports(5) works, but it's really easy for someone new to NFS to miss that they can't "safely" export a single subdirectory from a local filesystem on the server.

Go for it. I won't commit this one and will be happy to review
whatever you come up with.

I'm sorry for the delay. Here's my attempt: https://reviews.freebsd.org/D44614

[rmacklem]

markj@ has a better patch in D44614.