ihandle: OPEN fdPs are not counted in ihP refcount

ihandle: OPEN fdPs are not counted in ihP refcount

Just add a comment explaining that an OPEN FdHandle_t does not count
against the ref count for its parent IHandle_t. Recently I've seen
some confusion about this when discussing ihandle internals, and this
should make this abundantly clear.

Change-Id: I73210e0d05869827ba99d09f504052d1a6797669
Reviewed-on: http://gerrit.openafs.org/5317
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>