updated docs

docs/docs/faq.xml

@@ -120,6 +120,9 @@ row_vect.set_size(5);
             use them. However, CMake will try to link against them
             automatically if they are installed.</li><br/>
 
+            <li><b>FFTW</b> is used by the fft() and ifft() routines if you #define DLIB_USE_FFTW and
+            link to fftw3.  Otherwise dlib uses its own slower default implementation.  </li><br/>
+
             <li><b>SQLite</b> is used by the <a href="other.html#database">database</a> object.  In
             fact, it is just a wrapper around SQLite's C interface which simplifies its use (e.g. 
             makes resource management use RAII).</li>

docs/docs/linear_algebra.xml

@@ -390,6 +390,14 @@
          <item nolink="true">
             <name>Other_Utilities</name>
             <sub>
+               <item>
+                  <name>fft</name>
+                  <link>dlib/matrix/matrix_fft_abstract.h.html#fft</link>
+               </item>
+               <item>
+                  <name>ifft</name>
+                  <link>dlib/matrix/matrix_fft_abstract.h.html#ifft</link>
+               </item>
                <item>
                   <name>is_col_vector</name>
                   <link>dlib/matrix/matrix_utilities_abstract.h.html#is_col_vector</link>

docs/docs/term_index.xml

@@ -497,6 +497,9 @@
          <term file="dlib/matrix/matrix_utilities_abstract.h.html" name="is_vector"          include="dlib/matrix.h"/>
          <term file="dlib/matrix/matrix_utilities_abstract.h.html" name="is_row_vector"      include="dlib/matrix.h"/>
          <term file="dlib/matrix/matrix_utilities_abstract.h.html" name="is_col_vector"      include="dlib/matrix.h"/>
+         <term file="dlib/matrix/matrix_fft_abstract.h.html" name="fft"                      include="dlib/matrix.h"/>
+         <term file="dlib/matrix/matrix_fft_abstract.h.html" name="ifft"                     include="dlib/matrix.h"/>
+         <term file="dlib/matrix/matrix_fft_abstract.h.html" name="is_power_of_two"          include="dlib/matrix.h"/>
 
 
          <term file="dlib/matrix/matrix_la_abstract.h.html" name="lu_decomposition"          include="dlib/matrix.h"/>