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/intro.rst b/Doc/c-api/intro.rst
index 7ca8693..bae5ce1 100644
--- a/Doc/c-api/intro.rst
+++ b/Doc/c-api/intro.rst
@@ -326,7 +326,7 @@
 caller). When a function passes ownership of a reference on to its caller, the
 caller is said to receive a *new* reference.  When no ownership is transferred,
 the caller is said to *borrow* the reference. Nothing needs to be done for a
-borrowed reference.
+:term:`borrowed reference`.
 
 Conversely, when a calling function passes in a reference to an  object, there
 are two possibilities: the function *steals* a  reference to the object, or it