Following up a discussion on a PR here : https://github.com/python/cpython/pull/12238, There is need to make sure the C-API docs are clear about what the "main" interpreter is.

I should have added something like this earlier, but here are key points to consider covering:

* "main" interpreter is the original, created when the runtime initializes
* historically almost always the only Python interpreter in a process
    * this is changing (more projects using subinterpreters, PEP 554)
    * in the "python" executable, subinterpreters available via extension modules (but uncommon)
* it has extra responsiblities:
    * signal handling, in main thread
    * "pending calls", in main thread
        * not necessarily something to publicize :)
    * running in the "main" thread
        * can be taken over by a sub-interpreter, but not likely
    * execution *during* runtime initialization
    * usually (always?) the active interpreter during runtime finalization
    * others?
* no strong link between interpreters (e.g. main <--> sub <--> sub)
    * no record of what interpreter was active (if any) when a subinterpreter was created
    * no "parent" interpreter
    * thread states may share thread ID
* a bunch of stuff in the C-API and in the runtime still assumes that the main interpreter is running in the current OS thread
* other stuff?


Not all of that necessarily belongs there in the docs, but a lot of it does. :)

Thanks for this feedback. Let me see how I can incorporate this in the PR.

New changeset 854d0a4b98b13629252e21edaf2b785b429e5135 by Eric Snow (Joannah Nanjekye) in branch 'master':
bpo-36487: Make C-API docs  clear about what the main interpreter is. (gh-12666)
https://github.com/python/cpython/commit/854d0a4b98b13629252e21edaf2b785b429e5135

Thanks, Joannah!

New changeset 375f35be0688da0fc0f27afc4faea76590d7c24d by Eric Snow (Miss Islington (bot)) in branch '3.8':
bpo-36487: Make C-API docs clear about what the main interpreter is. (gh-15080)
https://github.com/python/cpython/commit/375f35be0688da0fc0f27afc4faea76590d7c24d