Add documentation for GerritStatusPush migration to generators that accept callback arguments

[p12tic]

The current proposed migration is to use BuildStatusGenerator, BuildStartEndStatusGenerator, BuildSetStatusGenerator. This however requires complete rewrite of user code. There's already GerritBuildSetStatusGenerator, GerritBuildStartStatusGenerator, GerritBuildEndStatusGenerator that offer much more direct migration path, but this is not documented. We should write documentation to make migration easier even if Gerrit*Generator classes are an interim solution.