Improve UI and documentation for "Ignore Attributes" in Owners slightly

Summary:
See PHI251. Ref T13137.

• Replace the perplexing text box with a checkbox that explains what it does.
• Mention this feature in the documentation.

Test Plan:

• Clicked/unclicked checkbox.
• Read documentation.
• Used an existing checkbox control in Slowvote to make sure I didn't break it.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13137

Differential Revision: https://secure.phabricator.com/D19433