gh-143578: Restore note about patchlevel.h (#143596)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: encukou

Doc/c-api/apiabiversion.rst

@@ -67,6 +67,8 @@ See :ref:`stable` for a discussion of API and ABI stability across versions.
 
    The Python version as a string, for example, ``"3.4.1a2"``.
 
+These macros are defined in :source:`Include/patchlevel.h`.
+
 
 Run-time version
 ----------------