Improve Customizer API documentation

[ericlewis]

In my work on the metadata UI API, I've found it helpful to document an existing, very related API: the Customizer API. I found it scant on inline documentation, and would like to offer what I have done already.

I was inspired a bit by wonderboymusic's work in #26870, and when he says "As a community, we need to help each other learn what it is and what it does." Feel you dog.

[ericlewis]

attachment:27065.diff​ is a start. Inline documentation is correct where invalid, and added onto where helpful in the server-side code of the Customizer API.

[nacin]

In 27398:

Update the Customizer API inline docs.

props ericlewis.
fixes #27065.