Add proper javadoc comments to core/lib/plugin_manager.js

[bmuenzenmeyer]

Code is read more than it is written.

Akin to work already done in core/lib/ui_builder.js

Read the code, and describe each function and parameters accordingly.

https://en.wikipedia.org/wiki/JSDoc

[yarwest]

@bmuenzenmeyer Hey! I'd love to document this file for you :) I have experience working with JS and JSDoc.