doc/filters: add mandelbrot source documentation

Address trac ticket #1492.

doc/filters.texi

@@ -3635,6 +3635,79 @@ cellauto=p='@@@@ @@ @@@@':s=100x400:full=0:rule=18
 
 @end itemize
 
+@section mandelbrot
+
+Generate a Mandelbrot set fractal, and progressively zoom towards the
+point specified with @var{start_x} and @var{start_y}.
+
+This source accepts a list of options in the form of
+@var{key}=@var{value} pairs separated by ":". A description of the
+accepted options follows.
+
+@table @option
+
+@item end_pts
+Set the terminal pts value. Default value is 400.
+
+@item end_scale
+Set the terminal scale value.
+Must be a floating point value. Default value is 0.3.
+
+@item inner
+Set the inner coloring mode, that is the algorithm used to draw the
+Mandelbrot fractal internal region.
+
+It shall assume one of the following values:
+@table @option
+@item black
+Set black mode.
+@item conververgence
+Show time until convergence.
+@item mincol
+Set color based on point closest to the origin of the iterations.
+@item period
+Set period mode.
+@end table
+
+Default value is @var{mincol}.
+
+@item bailout
+Set the bailout value. Default value is 10.0.
+
+@item maxiter
+Set the maximum of iterations performed by the rendering
+algorithm. Default value is 7189.
+
+@item outer
+Set outer coloring mode.
+It shall assume one of following values:
+@table @option
+@item iteration_count
+Set iteration cound mode.
+@item normalized_iteration_count
+set normalized iteration count mode.
+@end table.
+Default value is @var{normalized_iteration_count}.
+
+@item rate, r
+Set frame rate, expressed as number of frames per second. Default
+value is "25".
+
+@item size, s
+Set frame size. Default value is "640x480".
+
+@item start_scale
+Set the initial scale value. Default value is 3.0.
+
+@item start_x
+Set the initial x position. Must be a floating point value between
+-100 and 100. Default value is -0.743643887037158704752191506114774.
+
+@item start_y
+Set the initial y position. Must be a floating point value between
+-100 and 100. Default value is -0.131825904205311970493132056385139.
+@end table
+
 @section mptestsrc
 
 Generate various test patterns, as generated by the MPlayer test filter.