doc/filters: extend/fix documentation for the geq filter

doc/filters.texi

@@ -4004,25 +4004,31 @@ The filter accepts the following options:
 
 @table @option
 @item lum_expr
-the luminance expression
+Set the luminance expression.
 @item cb_expr
-the chrominance blue expression
+Set the chrominance blue expression.
 @item cr_expr
-the chrominance red expression
+Set the chrominance red expression.
 @item alpha_expr
-the alpha expression
+Set the alpha expression.
 @item r
-the red expression
+Set the red expression.
 @item g
-the green expression
+Set the green expression.
 @item b
-the blue expression
+Set the blue expression.
 @end table
 
+The colorspace is selected according to the specified options. If
+one of the @option{lum_expr}, @option{cb_expr}, or @option{cr_expr}
+options is specified, the filter will automatically select a YCbCr
+colorspace. If one of the @option{r}, @option{g}, or @option{b}
+options is specified, it will select an RGB colorspace.
+
 If one of the chrominance expression is not defined, it falls back on the other
 one. If no alpha expression is specified it will evaluate to opaque value.
-If none of chrominance expressions are
-specified, they will evaluate the luminance expression.
+If none of chrominance expressions are specified, they will evaluate
+to the luminance expression.
 
 The expressions can use the following variables and functions:
 
@@ -4058,15 +4064,21 @@ plane.
 
 @item cb(x, y)
 Return the value of the pixel at location (@var{x},@var{y}) of the
-blue-difference chroma plane. Returns 0 if there is no such plane.
+blue-difference chroma plane. Return 0 if there is no such plane.
 
 @item cr(x, y)
 Return the value of the pixel at location (@var{x},@var{y}) of the
-red-difference chroma plane. Returns 0 if there is no such plane.
+red-difference chroma plane. Return 0 if there is no such plane.
+
+@item r(x, y)
+@item g(x, y)
+@item b(x, y)
+Return the value of the pixel at location (@var{x},@var{y}) of the
+red/green/blue component. Return 0 if there is no such component.
 
 @item alpha(x, y)
 Return the value of the pixel at location (@var{x},@var{y}) of the alpha
-plane. Returns 0 if there is no such plane.
+plane. Return 0 if there is no such plane.
 @end table
 
 For functions, if @var{x} and @var{y} are outside the area, the value will be