[filter-effects-1] Represent invalidity of negative <length> in blur() in grammar

Doing this in the grammar rather than the prose makes it clearer

Author: Calme1709

filter-effects-1/Overview.bs

@@ -375,12 +375,10 @@ Unless defined otherwise, omitted values default to the <a for=svg>initial value
 Note: For some filter functions the default value for omitted values differes from their <a for=svg>initial value</a> for interpolation. For the convenience of content creators, the default value for omitted values for ''grayscale()'', ''sepia()'' and ''invert()'' is ''1'' (apply the effect to 100%) while the <a for=svg>initial value</a> for interpolation is ''0'' (no effect).
 
 <dl id="FilterFunction" dfn-for="filter">
-    : <pre class=prod><dfn>blur()</dfn> = blur( <<length>>? )</pre>
+    : <pre class=prod><dfn>blur()</dfn> = blur( <<length [0,∞]>>? )</pre>
     ::
         Applies a Gaussian blur to the input image. The passed parameter defines the value of the standard deviation to the Gaussian function. The parameter is specified a CSS length, but does not accept percentage values. The markup equivalent of this function is <a href="#blurEquivalent">given below</a>.
 
-        Negative values are not allowed.
-
         Default value when omitted is ''0px''.
 
         The <a for=svg>initial value</a> for interpolation is ''0px''.