Migrate AMI VarSet events raised by GoSub local variables

This patch moves VarSet events for local variables raised by GoSub
over to Stasis-Core. It also tweaks up the post-processing documentation
scripts to not combine parameters if both parameters are already documented.

(issue ASTERISK-21462)

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@387519 f38db490-d61c-443f-a65b-d21fe96a405b