STYLE: add usage information for subsetMesh argument

applications/utilities/mesh/manipulation/subsetMesh/subsetMesh.C

@@ -380,7 +380,14 @@ int main(int argc, char *argv[])
 
     #include "addOverwriteOption.H"
     #include "addRegionOption.H"
-    argList::addArgument("setOrZoneName");
+    argList::addArgument
+    (
+        "cell-selection",
+        "The cellSet name, but with the -zone option this is interpreted"
+        " to be a cellZone selection by name(s) or regex.\n"
+        "Eg 'mixer' or '( mixer \"moving.*\" )'"
+    );
+
     argList::addOption
     (
         "patch",