"Highly discourage" use of proto_tree_add_text.

Change-Id: I2e8d18df71688c654f7acaff51fae7823c08aa6a
Reviewed-on: https://code.wireshark.org/review/3677
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>
Reviewed-by: Evan Huus <eapache@gmail.com>