[3.14] gh-143578: Restore note about patchlevel.h (GH-143596) (#143725)

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Author: hugovk

Doc/c-api/apiabiversion.rst

@@ -46,6 +46,8 @@ See :ref:`stable` for a discussion of API and ABI stability across versions.
    Use this for numeric comparisons, for example,
    ``#if PY_VERSION_HEX >= ...``.
 
+These macros are defined in :source:`Include/patchlevel.h`.
+
 
 Run-time version
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