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

src/docs/user/userguide/owners.diviner

@@ -132,6 +132,21 @@ behavior. If you want more powerful auditing behavior, you can use Herald to
 write more sophisticated rules.
 
 
+Ignored Attributes
+==================
+
+You can automatically exclude certain types of files, like generated files,
+with **Ignored Attributes**.
+
+When a package is marked as ignoring files with a particular attribute, and
+a file in a particular change has that attribute, the file will be ignored when
+computing ownership.
+
+(This feature is currently rough, only works for Differential revisions, and
+may not always compute the correct set of owning packages in some complex
+cases where it interacts with dominion rules.)
+
+
 Files in Multiple Packages
 ==========================