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/reflection.rst b/Doc/c-api/reflection.rst
index 9207d86..64ce4d1 100644
--- a/Doc/c-api/reflection.rst
+++ b/Doc/c-api/reflection.rst
@@ -35,7 +35,7 @@
 
    Get the *frame* next outer frame.
 
-   Return a strong reference, or ``NULL`` if *frame* has no outer frame.
+   Return a :term:`strong reference`, or ``NULL`` if *frame* has no outer frame.
 
    *frame* must not be ``NULL``.
 
@@ -46,7 +46,7 @@
 
    Get the *frame* code.
 
-   Return a strong reference.
+   Return a :term:`strong reference`.
 
    *frame* must not be ``NULL``. The result (frame code) cannot be ``NULL``.