[manual] fftw unclear to me

[Francesco Potorti`]

>> Then I could ask myself: what are the FFTW libaries, and what should I
>> do to initialize them?
>>   
>If you don't understand what a library is, you shouldn't be using fftw 
>as it can do you more harm than good in the wrong hands..

I suspect many people using Octave have a very shallow knowledge of an
operating system's internals.  From another point of view, they are
using Octave, and the manual should allow them to understand how it
works.  If you think that my proposal makes things unnecessarily complex
I can try to reword it.

>I'm not sure of the process of appending a string from Octave to the 
>system-wide wisdom file. Frankly, if you want to do this you are better 
>off using the tool "fftw_wisdom" supplied with FFTW itself to manipulate 
>this file. I'd therefore rather not document this.

Or else simply add something that mentions the fftw-wisdom program.