Explain that the output record count may vary randomly.
A probability option, `--probability frac`, determines the probability that
an input record is passed to the standard output file. The probability ranges
-from 0.0 to 1.0, with 1 being the default.
+from 0.0 to 1.0, with 1 being the default. The sampling probaility is applied
+to each record (edge or node) in the input file independently. The number of
+records in the output file might not be exactly the same as the fraction times
+the number of records in the input file.
This command defaults to `--mode=NONE` since it doesn't attach special meaning
to particular columns.
