bpo-42294: Add borrowed/strong reference to doc glossary (GH-23206)

Add "borrowed reference" and "strong reference" to the documentation
glossary.

Enhance also Py_INCREF() and Py_NewRef() documentation.
diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 3ce6892..3d18bb3 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -1077,7 +1077,7 @@
 
    Get the current frame of the Python thread state *tstate*.
 
-   Return a strong reference. Return ``NULL`` if no frame is currently
+   Return a :term:`strong reference`. Return ``NULL`` if no frame is currently
    executing.
 
    See also :c:func:`PyEval_GetFrame`.