Add release notes for Review Board 2.0.11.

This covers the upgrade fixes and timezone fixes and the improved
support for showing Perforce changeset numbers,
Bump to 2.0.12 dev.
Minor documentation change to webapi resource model (File attachment/Screenshot comment)

1. /reviewboard/webapi/resources/file_attachment_comment.py: typo "filess"

2. /reviewboard/webapi/resources/review_file_attachment_comment.py: file
   attachment comments do not contain dimension infomation, while screenshot
   comments do have.

3. /reviewboard/webapi/resources/screenshot_comment.py: apparently these should
   be comments on a screenshot.

Testing Done:
No testing required.

Reviewed at https://reviews.reviewboard.org/r/6528/