runtime: expand comments on runtime panic checks

This adds comments explaining why it's important that some panics are
allowed in the runtime (even though this isn't ideal).

Change-Id: I04c6fc4f792f3793f951619ccaea6bfef2f1763c
Reviewed-on: https://go-review.googlesource.com/c/go/+/181737


Reviewed-by: Keith Randall <khr@golang.org>