Clarify usage of OPT by noting that if it is set the default values will be

left out.

Misc/SpecialBuilds.txt

@@ -2,7 +2,8 @@ This file describes some special Python build types enabled via
 compile-time preprocessor defines.
 
 It is best to define these options in the OPT environment variable;
-``OPT="-DPy_REF_DEBUG" ./configure``.
+``OPT="-DPy_REF_DEBUG" ./configure``.  If you want the default values of
+OPT to also be included you will need to add them in yourself manually.
 
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 Py_REF_DEBUG                                              introduced in 1.4