say when comm separator disappears

[jidanni]

In (info "(coreutils)comm invocation"), after:

      With no options, `comm' produces three-column output.  Column one
   contains lines unique to FILE1, column two contains lines unique to
   FILE2, and column three contains lines common to both files.  Columns
   are separated by a single TAB character.

      The options `-1', `-2', and `-3' suppress printing of the
   corresponding columns.  Also see *note Common options::.

Please add: If two of them are used, the separator is no longer needed
and will not be printed.