[manual] Add explanation on what wisdom is.

[Francesco Potortì]

This is the first of a series (if I manage to find the time) of
changesets against the latest source that replicate changesets I had
sent in the past three months but were never applied.

# HG changeset patch
# User Francesco Potortì <pot at gnu.org>
# Date 1230320032 -3600
# Node ID 60107c9637c2e4f9c44111d60dcc7c54daedf4bf
# Parent  2e777f5135a3b9718b1622fd63a655f9a8830ecc
Add explanation on what wisdom is.

diff -r 2e777f5135a3 -r 60107c9637c2 doc/ChangeLog
--- a/doc/ChangeLog	Thu Dec 25 12:31:07 2008 +0100
+++ b/doc/ChangeLog	Fri Dec 26 20:33:52 2008 +0100
@@ -1,3 +1,8 @@ 2008-12-23  David Bateman  <dbateman at fre
+2008-12-26  Francesco Potortì  <pot at gnu.org>
+
+	* interpreter/signal.txi (Signal Processing): Add explanation of
+	what wisdom is.
+
 2008-12-23  David Bateman  <dbateman at free.fr>
 
 	* interpreter/sparse.txi: Document the eigs and svds functions.
diff -r 2e777f5135a3 -r 60107c9637c2 doc/interpreter/signal.txi
--- a/doc/interpreter/signal.txi	Thu Dec 25 12:31:07 2008 +0100
+++ b/doc/interpreter/signal.txi	Fri Dec 26 20:33:52 2008 +0100
@@ -22,6 +22,18 @@
 @DOCSTRING(detrend)
 
 @DOCSTRING(fft)
+
+Octave uses 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)