[3.13] gh-141004: Document Py_UNICODE_{HIGH, LOW}_SURROGATE functions (GH-141019) (GH-141026)

gh-141004: Document `Py_UNICODE_{HIGH, LOW}_SURROGATE` functions (GH-141019)
(cherry picked from commit bfe54810c4)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>

Doc/c-api/unicode.rst

@@ -316,12 +316,22 @@ These APIs can be used to work with surrogates:
 
    Check if *ch* is a low surrogate (``0xDC00 <= ch <= 0xDFFF``).
 
+.. c:function:: Py_UCS4 Py_UNICODE_HIGH_SURROGATE(Py_UCS4 ch)
+
+    Return the high UTF-16 surrogate (``0xD800`` to ``0xDBFF``) for a Unicode
+    code point in the range ``[0x10000; 0x10FFFF]``.
+
+.. c:function:: Py_UCS4 Py_UNICODE_LOW_SURROGATE(Py_UCS4 ch)
+
+    Return the low UTF-16 surrogate (``0xDC00`` to ``0xDFFF``) for a Unicode
+    code point in the range ``[0x10000; 0x10FFFF]``.
+
 .. c:function:: Py_UCS4 Py_UNICODE_JOIN_SURROGATES(Py_UCS4 high, Py_UCS4 low)
 
    Join two surrogate code points and return a single :c:type:`Py_UCS4` value.
    *high* and *low* are respectively the leading and trailing surrogates in a
-   surrogate pair. *high* must be in the range [0xD800; 0xDBFF] and *low* must
-   be in the range [0xDC00; 0xDFFF].
+   surrogate pair. *high* must be in the range ``[0xD800; 0xDBFF]`` and *low* must
+   be in the range ``[0xDC00; 0xDFFF]``.
 
 
 Creating and accessing Unicode strings