Fix docs for MWNamespace::clearCaches() removal

Fix docs for MWNamespace::clearCaches() removal

Originally I had intended setMwGlobals() to magically reset the
namespace-related services if namespace-related settings were changed,
but Tim told me to remove it during review and I forgot to update the
release notes. Moving forward, everyone will need to reset services
after every config change to ensure correctness, and there's no point in
trying to reset specific services automatically as special cases.

I also moved the note to the 1.34 notes, since this missed the cutoff
and should not be backported.

Change-Id: Ib7cbdaef22a15ddfc7aaf99d0972b99d3cddc011