The FilterSourceFile comment block said that the filter lists use
regular expressions, but that is not specific enough because .search and
.match have different semantics. crrev.com/c/3679163 added a filter that
only matched on PRESUBMIT.py instead of all of them because of this
difference - better documentation might have avoided this.

The new documentation also makes it clear that Linux-style path
separators in the regex expressions are fine (they have been supported
since crrev.com/c/3609117).

Bug: 1309977
Change-Id: I49d37fbb93ef4a85f143d928a56af8ce22850c76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3889880
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Jesse McKenna <jessemckenna@google.com>