clarifying fftw wisdom

[Francesco Potorti`]

Here is a proposal for clarifying wat is fftw wisdom.

For some reason, Mercurial does not write my name in the changes, even
though I have this in ~/.hgrc:

================
[ui]
username = Francesco Potortì <pot at gnu.org>
================

Moreover, it does not include in the changeset the modifications I made
to the Changelog, so I add it here:

================
2008-09-24  Francesco Potortì  <pot at gnu.org>

	* interpreter/signal.txi (Signal Processing): Add explanation on
	what wisdom is.
================

# HG changeset patch
# User pot at tucano.isti.cnr.it
# Date 1222255457 -7200
# Node ID 060b397aa568620b450f92055c05b0d647a8ba31
# Parent  696dcf9ab9c8984042ff56b90ce02c5534f124cb
Add explanation on what wisdom is.

diff -r 696dcf9ab9c8 -r 060b397aa568 doc/interpreter/signal.txi
--- a/doc/interpreter/signal.txi	Wed Sep 24 13:24:00 2008 +0200
+++ b/doc/interpreter/signal.txi	Wed Sep 24 13:24:17 2008 +0200
@@ -22,6 +22,18 @@
 @DOCSTRING(detrend)
 
 @DOCSTRING(fft)
+
+Octave usese the FFTW libraries to perform FFT computations. When Octave
+starts up and initializes the FFTW libraries, they read a system wide
+file (on a Unix system, it is typically @file{/etc/fftw/wisdom}) that
+contains information useful to speed up FFT computations.  This
+information is called the @emph{wisdom}.  The system-wide file allows
+wisdom to be shared between all applications using the FFTW libraries.
+
+Use the @code{fftw} function to generate and save wisdom.  Using the
+utilities provided together with the FFTW libraries (@command{fftw-wisdom}
+on Unix systems), you can even add wisdom generated by Octave to the
+system-wide wisdom file.
 
 @DOCSTRING(fftw)