Improve job scheduler docs re 'idle' and 'charging' constraints

Make it clear that the job scheduler's "idle" constraint is not the
same as "doze" or "device idle," and has nothing to do with
"maintenance windows" or similar.

Also make it clear that the charging constraint is not the same
as simply being connected to some power source.

Test: m offline-sdk-docs
Change-Id: Ibcccf911740d3cfbceaf6cb27bb18318a4f735ca