Fix the close-submitted hook docs in the 2.0 release notes.
Fix the close-submitted hook docs in the 2.0 release notes.

These docs were incorrect, showing a URL that doesn't actually work.
It was missing the hosting service ID. I've updated the URL, added the
list of supported IDs, and genericified some text that was specifically
calling out GitHub/BitBucket.

Reviewed at https://reviews.reviewboard.org/r/5850/

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