Update gprof documentation

[Nick Clifton]

Hi Segher,

>>   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 - I have applied these changes to my local copy here.  I will
wait a few days to see if there any other comments and then check the
updated version in.

Cheers
        Nick