Customizer documentation spike

[ericlewis]

Gonna document some Koop magic here.

[ericlewis]

In attachment:29157.diff​,

add descriptions and JSDocs for a bunch of constructors/methods/functions in customize-loader.js and customize-base.js.

Move extend() to live in api.Class.extend for a straightforward code read; doesn't break anything because it's not available outside of the IIFE and was directly applied to api.Class.extend after its declaration.

[wonderboymusic]

In 29450:

Add some docs to Customizer JS.

Props ericlewis.
See #29157.

[ericlewis]

attachment:29157.2.diff​ is some minor constructor inheritance and clarification on server side code.

[wonderboymusic]

In 29451:

Add some docs to Customizer JS.

Props ericlewis.
See #29157.

[celloexpressions]

All patches here were committed in 4.0, let's continue (much-needed) Customizer docs work in new tickets for 4.1.