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/