Sort subscriber map alphabetically

I regret our previous unsorted version. This is nicer.

Change-Id: Ibc8a5bdfd2ef33f7128eec4004c5e11697f6ae7b
Add some documentation to the start of subscriber_map

Change-Id: I45b8150a14e9fddec11bbb1d843a5993210c42a3
Merge "Sort subscriber map alphabetically"
Merge "Add some documentation to the start of subscriber_map"