Prevent 'docs' builder from starting unless there is a change in doc/.

We only want to see new generated documentation immediately for changes to doc/; for changes elsewhere, we can wait a few days.

Note: completely untested! Test with the buildbot check config tool?

More detail: the docs builder is currently scheduled to build every commit via a GitHub post-receive hook, but also to build every three days. For the 'every commit' scheduler, this change should cause builds to start only if there is a change in doc/.