avfilter/vf_palette(gen|use): support palettes with alpha

doc/filters.texi

@@ -16580,6 +16580,9 @@ Compute new histogram for each frame.
 @end table
 
 Default value is @var{full}.
+@item use_alpha
+Create a palette of colors with alpha components.
+Setting this, will automatically disable 'reserve_transparent'.
 @end table
 
 The filter also exports the frame metadata @code{lavfi.color_quant_ratio}
@@ -16658,6 +16661,11 @@ will be treated as completely opaque, and values below this threshold will be
 treated as completely transparent.
 
 The option must be an integer value in the range [0,255]. Default is @var{128}.
+
+@item use_alpha
+Apply the palette by taking alpha values into account. Only useful with
+palettes that are containing multiple colors with alpha components.
+Setting this will automatically disable 'alpha_treshold'.
 @end table
 
 @subsection Examples