Parse "comment" attributes in xr.xml for docstrings...

[cmbruns]

... and message strings for XrResult based exceptions.

This is not a full solution to #21 because there are no docstrings for the commands/functions in xr.xml. Where does that text come from?!

[cmbruns]

• [x] result exceptions
• [ ] enum values
• [ ] extension result exceptions
• [ ] XrObjectType mapping (not really docstrings...)