improve usage string

Hi LLDB folks,
It would be nice if the usage message mentioned the " lldb – a.out 1 2 3" variant.

In current TOT, the bottom of the usage says:

  (If you don't provide -f then the first argument will be the file to be debugged
   so 'lldb -- <filename> [<ARG1> [<ARG2>]]' also works.
   Remember to end the options with "--" if any of your arguments have a "-" in them.)