Update documentaion for p_(add|get)_proto_data(); There's been a 'key' arg for a while.

Change-Id: I1166a63d8896d0d5fc63b558ebb0df2e1e19bc63
Reviewed-on: https://code.wireshark.org/review/3197
Reviewed-by: Bill Meier <wmeier@newsguy.com>