Clarify what the C API contains

[encukou]

It's not clear what exactly is part of the public C API.
There are efforts to clarify, document and categorize, but they're ongoing. (i.e. the docs/devguide is good for new additions, but for the decades-old stuff, take it with a grain of salt.)

[encukou]

Note that there are well-defined subsets -- the limited API and the unstable API.
My solution/goal is to improve limited API and gradually let the rest slide into irrelevance, deprecation & removal -- but that will take decades, so it's too slow for stuff that blocks development of new features.