support omission of standard event listen example

support omission of standard event listen example

The required targets for before_configured() and after_configured()
are the Mapper (and soon to include registry things as well in 2.1),
update the automatic doc example thing to be able to be skipped
when there are special instructions for the target.

Also updates the event docs a bit, which were very old and
written in more of that "disorganized wall of details" style
that was very hard to unlearn

Change-Id: I729d08e25b721f4c993aa1856038b27643fc95d0
(cherry picked from commit 3544d89d2de0e910afc3648fe1d34c96900b101b)