Created attachment 287368 [details] [review]
proposed patch

According to the documentation, ATK_STATE_INDETERMINATE is specifically to indicate a tri-state checkbox is partially checked. That seems to be awfully specific and prevents it from addressing issues such as a large data set whose size is unknown and cannot (easily, immediately) be known. Example: Gmail's "1 of about 120". We don't need new API for the set size; we need docs that give implementors permission to use this state for that case.

I'm attaching a patch which adds that case as an example, and trims what seems like an unnecessary amount of explanation for the tri-state case.