gapit verb help text should describe positional arguments

Currently only flags are documented, e.g. `gapit benchmark` returns

```
GAPIT: GAPIT is a command line tool for the graphics api debugger system.
benchmark: Runs a set of benchmarking tests on an application
Usage: GAPIT benchmark [benchmark-flags]
benchmark-flags:
-additionalargs string
additional arguments to pass to the application
[...]
```

This says nothing about the positional argument of the verb, in that case the name of the process to run the benchmark on.

Also, running `gapit ` seem to print the help for that verb, this is not working with `benchmark`.