be more precise about rig list source

git-svn-id: https://hamlib.svn.sourceforge.net/svnroot/hamlib/trunk@1860 7ae35d74-ebe9-4afe-98af-79ac388436b8

tests/rigmem.1

@@ -37,7 +37,7 @@ options starting with two dashes (`-').
 A summary of options is included below.
 .TP
 .B \-m, \-\-model=id
-Select radio model number. See model list.
+Select radio model number. See model list, as reported by rigctl.
 .TP
 .B \-r, --rig-file=device
 Use \fBdevice\fP as the file name of the radio to operate on.