[selectors] Forgot to allow and define the case where :drop() is empty.

Author: tabatkins

selectors/Overview.html

@@ -3003,7 +3003,7 @@ <h3 id=drag-pseudos><span class=secno>9.4. </span> The drag-and-drop
    additional filters to be specified that can exclude some drop targets. Its
    syntax is:
 
-  <pre class=prod>:drop( [ active || valid || invalid ] )</pre>
+  <pre class=prod>:drop( [ active || valid || invalid ]? )</pre>
 
   <p> The keywords have the following meanings:
 
@@ -3035,6 +3035,11 @@ <h3 id=drag-pseudos><span class=secno>9.4. </span> The drag-and-drop
    class=css>:drop(valid active)</code>’ will match the active drop target
    <em>if</em> it's valid, but not if it's invalid.
 
+  <p> If no keywords are given in the argument, ‘<a href="#drop0"><code
+   class=css>:drop()</code></a>’ has the same meaning as ‘<a
+   href="#drop"><code class=css>:drop</code></a>’ - it matches every drop
+   target.
+
   <h2 id=time-pseudos><span class=secno>10. </span> Time-dimensional
    Pseudo-classes</h2>
 

selectors/Overview.src.html

@@ -2050,7 +2050,7 @@ <h3 id="drag-pseudos">
     but allows additional filters to be specified that can exclude some drop targets.
     Its syntax is:
 
-  <pre class='prod'>:drop( [ active || valid || invalid ] )</pre>
+  <pre class='prod'>:drop( [ active || valid || invalid ]? )</pre>
 
   <p>
     The keywords have the following meanings:
@@ -2084,6 +2084,11 @@ <h3 id="drag-pseudos">
     For example, '':drop(valid active)'' will match the active drop target <em>if</em> it's valid,
     but not if it's invalid.
 
+  <p>
+    If no keywords are given in the argument,
+    '':drop()'' has the same meaning as '':drop''
+    - it matches every drop target.
+
 <h2 id="time-pseudos">
 Time-dimensional Pseudo-classes</h2>