doc: dynamic mount requires dynroot and fakestat

Document that dynamic mount (/afs/.:mount) requires
dynroot and fakestat on non-linux unix.

Change-Id: I947edd30d510c7cc6840bc2cc74d0ef07b692afb
Reviewed-on: http://gerrit.openafs.org/8739
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>