fix bug in that comments/text are str not Comment/Text

These commits including (a) a very minor fix for the issueabout comments/text  (b) some additional tests and (c) some documentation improvements.

The bugs in (a) makes it impossible to take advantage of the deleted attribute for both. Ideally, we should also include examples of deleted revisions into the test data so that we can identify these attributes in the tests. Because I didn't have examples of this offhand, I didn't add examples of deleted comments/reviseions to the test data.