doc/filters: document testsrc2 source filter

Signed-off-by: Lou Logan <lou@lrcd.com>

doc/filters.texi

@@ -14330,7 +14330,8 @@ ffplay -f lavfi life=s=300x200:mold=10:r=60:ratio=0.1:death_color=#C83232:life_c
 @anchor{smptebars}
 @anchor{smptehdbars}
 @anchor{testsrc}
-@section allrgb, allyuv, color, haldclutsrc, nullsrc, rgbtestsrc, smptebars, smptehdbars, testsrc
+@anchor{testsrc2}
+@section allrgb, allyuv, color, haldclutsrc, nullsrc, rgbtestsrc, smptebars, smptehdbars, testsrc, testsrc2
 
 The @code{allrgb} source returns frames of size 4096x4096 of all rgb colors.
 
@@ -14359,6 +14360,10 @@ The @code{testsrc} source generates a test video pattern, showing a
 color pattern, a scrolling gradient and a timestamp. This is mainly
 intended for testing purposes.
 
+The @code{testsrc2} source is similar to testsrc, but supports more
+pixel formats instead of just @code{rgb24}. This allows using it as an
+input for other tests without requiring a format conversion.
+
 The sources accept the following parameters:
 
 @table @option