Extensions: Add documentation to content_scripts.idl.

Follow-up from
https://chromium-review.googlesource.com/c/chromium/src/+/2419607. This
documentation is not yet exposed on the docserver.

BUG=113513

Change-Id: I164efcf67789e699b35322d1a5b6e519147944b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519041


Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824837}