Improve the Plugins documentation for referencability

Updates the reference section of the plugin documentation as follows:

1. Split into two sub-sections: attributes and methods
2. Alphabetize the entries in each section
3. Cross-reference each entry with related via "See also:..."

Update icon_src() and icon_atl() with examples and rationale.

I'll leave this unmerged so that it can be reviewed by others for a few hours.