Does anyone happen to have created or found anywhere, a PyMel Class Diagram…with all the classes that currently exist in PyMel?
Not a diagram, but you can do a blank search in the PyMEL docs. 
http://download.autodesk.com/global/docs/maya2012/en_us/PyMel/search.html?q=&check_keywords=yes&area=default
(warning, it will take a while to load, hehe)
You can also search using dots, so pymel.core.datatypes is valid which lets you drill down into any of the classes or methods.
I don’t know if there is actually an answer to your question, but as a related tip, I did set up a custom bookmark in Firefox for quickly searching the PyMEL docs. I find it fairly easy to browse the docs this way.
Replace the search term in a search results URL with %s and then assign it a keyword such as “pm”.
Here are instructions. You can do it in Chrome as well. How to Add, Create and Manage Search Engines in Firefox - Make Tech Easier
And then all I have to do is type “pm matrix” or whatever, and it will bring me straight to the docs.