gas: minor reformatting in command line help and doc

- help message: add a comma between the short and long option
- as doc:
  - brief summary of how to invoke gas: separate [-w] [-x] on a new line as those
  two options have nothing to do with the warning options.
  - reordering of the warning options to have the same order as the listing.
  - no-warn option description: change an "and" to a "or", as it is either the short
  or long option to use, but not both at the same time.
- remove trailing whitespaces.