Add documentation for test_that

This used to live on the team site, but that documentation is becoming
increasingly out of date as we add new features and remove old ones.
Fight documentation age by moving user visible docs to the codebase
itself.

BUG=b:26912592
TEST=None

Change-Id: I98796bdec26b325bd10065566a98edc4d6f5c42a
Reviewed-on: https://chromium-review.googlesource.com/325074
Commit-Ready: Christopher Wiley <wiley@chromium.org>
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
1 file changed