doc: clarify that testnet min-difficulty is not optional

src/pow.cpp

@@ -23,7 +23,7 @@ unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHead
         {
             // Special difficulty rule for testnet:
             // If the new block's timestamp is more than 2* 10 minutes
-            // then allow mining of a min-difficulty block.
+            // then it MUST be a min-difficulty block.
             if (pblock->GetBlockTime() > pindexLast->GetBlockTime() + params.nPowTargetSpacing*2)
                 return nProofOfWorkLimit;
             else