Add and document PARKEDCALL variable set during timeout

Add and document PARKEDCALL variable set during timeout

PARKEDCALL variable tracks which parking lot the call was last parked in.  This can be
used afterwards for flow control when returntoorigin is set to off. I went ahead and
documented both this and the existing variable set during timeout (PARKINGSLOT) in
the sample features.conf since there was no prior mention of variables being set during
timeout.

(closes issue ASTERISK-16239)
Reported By: Clod Patry
Patches:
	M17503.diff uploaded by Clod Patry (license 5138)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@348161 65c4cc65-6c06-0410-ace0-fbb531ad65f3