doc: Update outdated txnouttype documentation

Also, remove scope of txnouttype in fuzz tests temporarily. The next
commit will add scopes to all txnouttype.

src/script/standard.cpp

@@ -56,7 +56,7 @@ std::string GetTxnOutputType(txnouttype t)
     case TX_WITNESS_V0_KEYHASH: return "witness_v0_keyhash";
     case TX_WITNESS_V0_SCRIPTHASH: return "witness_v0_scripthash";
     case TX_WITNESS_UNKNOWN: return "witness_unknown";
-    }
+    } // no default case, so the compiler can warn about missing cases
     assert(false);
 }