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.h

@@ -543,6 +543,10 @@ public:
             opcodetype opcode;
             if (!GetOp(pc, opcode))
                 return false;
+            // Note that IsPushOnly() *does* consider OP_RESERVED to be a
+            // push-type opcode, however execution of OP_RESERVED fails, so
+            // it's not relevant to P2SH as the scriptSig would fail prior to
+            // the P2SH special validation code being executed.
             if (opcode > OP_16)
                 return false;
         }