Add a section describing PyUnstable_GetDefaultInterpreterRef's motivation.

Author: ZeroIntensity

peps/pep-0788.rst

@@ -387,6 +387,21 @@ The exact details of this deprecation aren't too clear. It's likely that
 the usual five-year deprecation (as specificed by :pep:`387`) will be too
 short, so for now, these functions will have no specific removal date.
 
+Compatibility Shim for ``PyGILState_Ensure``
+--------------------------------------------
+
+This proposal comes with :c:func:`PyUnstable_GetDefaultInterpreterRef` as a
+compatibility hack for some users of :c:func:`PyGILState_Ensure`. It is a
+thread-safe way to acquire a strong reference to the main (or "default")
+interpreter.
+
+The main drawback to porting new code to :c:func:`PyThreadState_Ensure` is that
+it isn't a drop-in replacement for :c:func:`!PyGILState_Ensure`, as it needs
+an interpreter reference argument. In some large applications, refactoring to
+use a :c:type:`PyInterpreterRef` everywhere might be tricky; so, this function
+acts as a silver bullet for users who explicitly want to disallow support for
+subinterpreters.
+
 Specification
 =============
 
@@ -906,7 +921,7 @@ Example: Calling Python Without a Callback Parameter
 There are a few cases where callback functions don't take a callback parameter
 (``void *arg``), so it's impossible to acquire a reference to any specific
 interpreter. The solution to this problem is to acquire a reference to the main
-interpreter through :c:func:`PyUnstable_InterpreterRef_GetMain`.
+interpreter through :c:func:`PyUnstable_GetDefaultInterpreterRef`.
 
 But wait, won't that break with subinterpreters, per
 :ref:`pep-788-subinterpreters-gilstate`? Fortunately, since the callback has
@@ -919,7 +934,7 @@ interpreter here.
     static void
     call_python(void)
     {
-        PyInterpreterRef ref = PyUnstable_InterpreterRef_GetMain();
+        PyInterpreterRef ref = PyUnstable_GetDefaultInterpreterRef();
         if (ref == 0) {
             fputs("Python has shut down.", stderr);
             return;