minor fixes from #3222 (#3368)
* clean up handling of aux data in jvp_subtrace_aux
related to #3222 indirectly, in that we now won't try to do something
crazy like `JVPTracer(trace, object(), zero)`, which #3222 didn't like
* symbolic zeros tweak to work with div rule
* fix a couple ad_util.Zero type checks
* fix a docs bug
* improve broadcast handling in batching.py
We can avoid a circular import just by `import jax`!
* fix another docs bug
* yet another doc fix
