Add a code comment for checking the maximum number of steps, and
disambiguate the different types of breakpoints.

R=thibaudm@chromium.org

Change-Id: I9be40461554948a61e81b3f9953cfc4475e52e54
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2985400
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75383}