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