Print warning about unsorted custom dumps without atom IDs. Explain in manual.

src/dump_custom.cpp

@@ -1245,11 +1245,14 @@ int DumpCustom::parse_fields(int narg, char **arg)
 {
   // customize by adding to if statement
 
+  has_id = 0;
+
   for (int iarg = 0; iarg < narg; iarg++) {
     if (strcmp(arg[iarg],"id") == 0) {
       pack_choice[iarg] = &DumpCustom::pack_id;
       if (sizeof(tagint) == sizeof(smallint)) vtype[iarg] = Dump::INT;
       else vtype[iarg] = Dump::BIGINT;
+      has_id = 1;
     } else if (strcmp(arg[iarg],"mol") == 0) {
       if (!atom->molecule_flag)
         error->all(FLERR,"Dumping an atom property that isn't allocated");