doc/filters: Add colourspace values for colormatrix filter

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Kieran Kunhya <kierank@obe.tv>

doc/filters.texi

@@ -5260,15 +5260,24 @@ The accepted values are:
 @item bt709
 BT.709
 
+@item fcc
+FCC
+
 @item bt601
 BT.601
 
+@item bt470
+BT.470
+
+@item bt470bg
+BT.470BG
+
+@item smpte170m
+SMPTE-170M
+
 @item smpte240m
 SMPTE-240M
 
-@item fcc
-FCC
-
 @item bt2020
 BT.2020
 @end table