bpo-36487: Make C-API docs clear about what the main interpreter is. (gh-12666)

Doc/c-api/init.rst

@@ -1141,10 +1141,18 @@ Sub-interpreter support
 
 While in most uses, you will only embed a single Python interpreter, there
 are cases where you need to create several independent interpreters in the
-same process and perhaps even in the same thread.  Sub-interpreters allow
-you to do that.  You can switch between sub-interpreters using the
-:c:func:`PyThreadState_Swap` function.  You can create and destroy them
-using the following functions:
+same process and perhaps even in the same thread. Sub-interpreters allow
+you to do that.
+
+The "main" interpreter is the first one created when the runtime initializes.
+It is usually the only Python interpreter in a process.  Unlike sub-interpreters,
+the main interpreter has unique process-global responsibilities like signal
+handling.  It is also responsible for execution during runtime initialization and
+is usually the active interpreter during runtime finalization.  The
+:c:func:`PyInterpreterState_Main` funtion returns a pointer to its state.
+
+You can switch between sub-interpreters using the :c:func:`PyThreadState_Swap`
+function. You can create and destroy them using the following functions:
 
 
 .. c:function:: PyThreadState* Py_NewInterpreter()

Misc/NEWS.d/next/Documentation/2019-04-02-19-23-00.bpo-36487.Jg6-MG.rst

+Make C-API docs clear about what the "main" interpreter is.
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