[3.14] gh-141004: Document pyctype.h macros (GH-141272) (GH-141340)

gh-141004: Document `pyctype.h` macros (GH-141272)
(cherry picked from commit df19261621)

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

Doc/c-api/conversion.rst

@@ -172,3 +172,58 @@ The following functions provide locale-independent string to number conversions.
 
    Case insensitive comparison of strings. The function works almost
    identically to :c:func:`!strncmp` except that it ignores the case.
+
+
+Character classification and conversion
+=======================================
+
+The following macros provide locale-independent (unlike the C standard library
+``ctype.h``) character classification and conversion.
+The argument must be a signed or unsigned :c:expr:`char`.
+
+
+.. c:macro:: Py_ISALNUM(c)
+
+   Return true if the character *c* is an alphanumeric character.
+
+
+.. c:macro:: Py_ISALPHA(c)
+
+   Return true if the character *c* is an alphabetic character (``a-z`` and ``A-Z``).
+
+
+.. c:macro:: Py_ISDIGIT(c)
+
+   Return true if the character *c* is a decimal digit (``0-9``).
+
+
+.. c:macro:: Py_ISLOWER(c)
+
+   Return true if the character *c* is a lowercase ASCII letter (``a-z``).
+
+
+.. c:macro:: Py_ISUPPER(c)
+
+   Return true if the character *c* is an uppercase ASCII letter (``A-Z``).
+
+
+.. c:macro:: Py_ISSPACE(c)
+
+   Return true if the character *c* is a whitespace character (space, tab,
+   carriage return, newline, vertical tab, or form feed).
+
+
+.. c:macro:: Py_ISXDIGIT(c)
+
+   Return true if the character *c* is a hexadecimal digit (``0-9``, ``a-f``, and
+   ``A-F``).
+
+
+.. c:macro:: Py_TOLOWER(c)
+
+   Return the lowercase equivalent of the character *c*.
+
+
+.. c:macro:: Py_TOUPPER(c)
+
+   Return the uppercase equivalent of the character *c*.