Include export/import macro in staticToken documentation

Author: nvmkuruc

pxr/base/tf/staticTokens.h

@@ -29,7 +29,10 @@
 ///        // Syntax when string name differs from symbol.
 ///        ((foo, "bar"))
 ///
-///    TF_DECLARE_PUBLIC_TOKENS(MfTokens, MF_TOKENS);
+///    // Include an export/import API macro as the second argument if you need
+///    // the symbols to be exported. Othwerise, the two argument version
+///    // `TF_DECLARE_PUBLIC_TOKENS(MfTokens, MF_TOKENS);` is sufficient.
+///    TF_DECLARE_PUBLIC_TOKENS(MfTokens, MF_API, MF_TOKENS);
 /// \endcode
 ///
 /// In cpp file: