HOWTO: update command line option descriptions

HOWTO: update command line option descriptions

- Make it clearer what --help will print (since page is ambiguous in
  the context of the HOWTO).
- Update the wording and syntax for --crctest so it's clear it doesn't
  always need a parameter.
- Use KiB rather than kb to refer to powers of two byte sizes and update
  the maximum number of pools.
- Make the --idle-prof description more verbose (and clear up confusion
  that might make people try and pass it literal option
  'system,percpu').

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>