Document and test OP_RESERVED weirdness

Seems it was forgotten about when IsPushOnly() and the unittests were
written. A particular oddity is that OP_RESERVED doesn't count towards
the >201 opcode limit unlike every other named opcode.

src/script.cpp

@@ -327,6 +327,8 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, co
                 return false;
             if (vchPushValue.size() > MAX_SCRIPT_ELEMENT_SIZE)
                 return false;
+
+            // Note how OP_RESERVED does not count towards the opcode limit.
             if (opcode > OP_16 && ++nOpCount > 201)
                 return false;