Bug 1403519 - only build docs when necessary r=gps

This marks **/docs/** as exclusively docs, and code that is autodoc'd as
inclusively docs.

That means that a change that purely modifies documentation files will *only*
run `docs` tasks, while a change that modifies autodoc'd source code will
*additionaly* run `docs` tasks.  The tasks do not run by default.

MozReview-Commit-ID: G9tOK0AwtrI

toolkit/components/url-classifier/moz.build

@@ -93,4 +93,7 @@ if CONFIG['NIGHTLY_BUILD'] or CONFIG['MOZ_DEBUG']:
 
 SPHINX_TREES['url-classifier'] = 'docs'
 
+with Files('docs/**'):
+    SCHEDULES.exclusive = ['docs']
+
 include('/ipc/chromium/chromium-config.mozbuild')