proresenc: alpha coding support

doc/encoders.texi

@@ -608,6 +608,7 @@ Select the ProRes profile to encode
 @item lt
 @item standard
 @item hq
+@item 4444
 @end table
 
 @item quant_mat @var{integer}
@@ -637,6 +638,11 @@ Override the 4-byte vendor ID.
 A custom vendor ID like @var{apl0} would claim the stream was produced by
 the Apple encoder.
 
+@item alpha_bits @var{integer}
+Specify number of bits for alpha component.
+Possible values are @var{0}, @var{8} and @var{16}.
+Use @var{0} to disable alpha plane coding.
+
 @end table
 
 @subsection Speed considerations