Update documentation and error message

m-core.h

@@ -4514,7 +4514,7 @@ m_core_parse2_enum (const char str[], const char **endptr)
   M_IF_OPLIST(oplist)(M_LETI3, M_LETI2_FAILURE)(cont, oplist, __VA_ARGS__)
 // Stop with a failure (invalid oplist)
 #define M_LETI2_FAILURE(cont, oplist, ...)                                    \
-  M_STATIC_ASSERT(false, M_LIB_NOT_AN_OPLIST, "(M_LET): the given argument is not a valid oplist: " M_AS_STR(oplist))
+  M_STATIC_ASSERT(false, M_LIB_NOT_AN_OPLIST, "the last argument of M_LET is not a valid oplist or a registered type: " M_AS_STR(oplist))
 // 4. Map all variables to their own LET
 #define M_LETI3(cont, oplist, ...)                                            \
   for(bool cont = true; cont ; /* unused */)                                  \