diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 21fa2be..b0c13b9 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -5147,14 +5147,32 @@ The delimiter @var{str} may not be empty.
 @item --total
 Output a summary at the end.
 
+Similar to the regular output,
+column one contains the total number of lines unique to @var{file1},
+column two contains the total number of lines unique to @var{file2}, and
+column three contains the total number of lines common to both files,
+followed by the word @samp{total} in the additional column four.
+
 In the following example, @command{comm} omits the regular output
 (@option{-123}), thus just printing the summary:
 
 @example
-$ printf '%s\n' 1 2 3 4     > file1
-$ printf '%s\n'   2 3 4 5 6 > file2
+$ printf '%s\n' a b c d e     > file1
+$ printf '%s\n'   b c d e f g > file2
 $ comm --total -123 file1 file2
-1       2       3       total
+1       2       4       total
address@hidden example
+
+This option is a GNU extension.  Portable scripts should use @command{wc} to
+get the totals, e.g. for the above example files:
+
address@hidden
+$ comm -23 file1 file2 | wc -l    # number of lines only in file1
+1
+$ comm -13 file1 file2 | wc -l    # number of lines only in file2
+2
+$ comm -12 file1 file2 | wc -l    # number of lines common to both files
+4
 @end example
 
 @optZeroTerminated