DOC: Added/fixed some docstrings, esp Panel flex

1. Panel flex docstrings had str representation of functions like
   `<builtin-func add..>` (not pretty). Fixed to be name
2. Added some docstrings to miscellaneous properties and clarified
   meaning of info_axis/keys.
3. Wrap 0.13's release.rst at 80 characters per line.

I'm happy to revert (3) above...just was bothering me (most of the doc
breaks at 80)