Clarify that 16777229 is the AppID for 3GPP Rx Release 6.

(Later releases have a different AppID.)

Change-Id: I41d4a498461e777f570641936db64d0551e36186
Reviewed-on: https://code.wireshark.org/review/4070
Reviewed-by: Anders Broman <a.broman58@gmail.com>