Update gprof documentation

[Segher Boessenkool]

Nick Clifton wrote:
>   Ok - how about this additional text to clear up the description
>   about gcc command line options ?

A lot better, thanks!

>   Note: The @samp{-pg} option must be part of your compilation options
> ! as well as your link options.  If it is not then only limited
> ! profiling information will be gathered

Might as well say exactly what data is not gathered, something like

"If it is not then no call-graph data will be gathered"

> + If you add the @samp{-Q} switch to suppress the printing of the call
> + graph data you will still be able to see some time samples:

s/some/the/ .


Thanks,

Segher