Document dropbox permissions

Document dropbox permissions

Document the behavior and potential problems with granting 'il' rights
to create dropboxes. Do this in the manpage for 'fs setacl' and
chapter 4 of the User Guide.

Reviewed-on: http://gerrit.openafs.org/3977
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 9c7c6a2ca5c45367d1a8ed1710a4c99f8e3c5f6b)

Change-Id: I9514087016a2946934d253a79ce0721ce1d8ea0f
Reviewed-on: http://gerrit.openafs.org/4087
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>