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

python/moz.build

@@ -37,8 +37,14 @@ SPHINX_PYTHON_PACKAGE_DIRS += [
     'mozversioncontrol/mozversioncontrol',
 ]
 
+with Files('**.py'):
+    SCHEDULES.inclusive += ['docs']
+
 SPHINX_TREES['mach'] = 'mach/docs'
 
+with Files('mach/docs/**'):
+    SCHEDULES.exclusive = ['docs']
+
 PYTHON_UNITTEST_MANIFESTS += [
     'mach/mach/test/python.ini',
     'mozbuild/dumbmake/test/python.ini',

python/mozbuild/mozbuild/schedules.py

@@ -11,7 +11,12 @@ from __future__ import absolute_import, unicode_literals, print_function
 
 # TODO: ideally these lists could be specified in moz.build itself
 
+# Inclusive components are those which are scheduled when certain files are
+# changed, but do not run by default.  These are generally added to
+# `SCHEDULES.inclusive` using `+=`, but can also be used as exclusive
+# components for files which *only* affect the named component.
 INCLUSIVE_COMPONENTS = [
+    'docs',
     'py-lint',
     'js-lint',
     'yaml-lint',
@@ -23,6 +28,10 @@ INCLUSIVE_COMPONENTS = [
 ]
 INCLUSIVE_COMPONENTS = sorted(INCLUSIVE_COMPONENTS)
 
+# Exclusive components are those which are scheduled by default, but for which
+# some files *only* affect that component.  For example, most files affect all
+# platforms, but platform-specific files exclusively affect a single platform.
+# These components are assigned to `SCHEDULES.exclusive` with `=`.
 EXCLUSIVE_COMPONENTS = [
     # os families
     'android',