gh-141004: Document Py_BUILD_ASSERT* macros (GH-141266)

Doc/c-api/intro.rst

@@ -233,6 +233,29 @@ complete listing.
 
    .. versionadded:: 3.4
 
+.. c:macro:: Py_BUILD_ASSERT(cond)
+
+   Asserts a compile-time condition *cond*, as a statement.
+   The build will fail if the condition is false or cannot be evaluated at compile time.
+
+   For example::
+
+      Py_BUILD_ASSERT(sizeof(PyTime_t) == sizeof(int64_t));
+
+   .. versionadded:: 3.3
+
+.. c:macro:: Py_BUILD_ASSERT_EXPR(cond)
+
+   Asserts a compile-time condition *cond*, as an expression that evaluates to ``0``.
+   The build will fail if the condition is false or cannot be evaluated at compile time.
+
+   For example::
+
+      #define foo_to_char(foo) \
+          ((char *)(foo) + Py_BUILD_ASSERT_EXPR(offsetof(struct foo, string) == 0))
+
+   .. versionadded:: 3.3
+
 .. c:macro:: PyDoc_STRVAR(name, str)
 
    Creates a variable with name *name* that can be used in docstrings.

Doc/conf.py

@@ -226,9 +226,6 @@ nitpick_ignore = [
 # Temporary undocumented names.
 # In future this list must be empty.
 nitpick_ignore += [
-    # Undocumented public C macros
-    ('c:macro', 'Py_BUILD_ASSERT'),
-    ('c:macro', 'Py_BUILD_ASSERT_EXPR'),
     # Do not error nit-picky mode builds when _SubParsersAction.add_parser cannot
     # be resolved, as the method is currently undocumented. For context, see
     # https://github.com/python/cpython/pull/103289.