Add more docs on parens. Fixes peggyjs#398.

docs/documentation.html

@@ -753,7 +753,12 @@ <h3 id="grammar-syntax-and-semantics-parsing-expression-types">Parsing Expressio
   <dt><code>( <em>expression</em> )</code></dt>
 
   <dd>
-    <p>Match a subexpression and return its match result.</p>
+    <p>Match a subexpression and return its match result.  Parentheses create
+    a new local context for the <a href="#action-execution-environment">Action
+    Execution Environment</a> as well as plucks with the <code>@</code>
+    operator.  Note that the action block in the following example returns
+    <code>2</code> from the parenthesized expression, NOT from the rule -- the
+    rule returns an array of <code>2</code>'s due to the <code>+</code> operator.</p>
 
     <div class="example">
       <div>
@@ -767,6 +772,22 @@ <h3 id="grammar-syntax-and-semantics-parsing-expression-types">Parsing Expressio
         <div class="result"></div>
       </div>
     </div>
+
+    <p>Similarly, in the next example, the pluck operator applies to the
+    return value of the parentheses, not the rule:</p>
+
+    <div class="example">
+      <div>
+        <div><em>Example:</em> <code>paren_pluck = (@[0-9] ",")+</code></div>
+        <div><em>Matches:</em> <code>"1,"</code>, <code>"2,3,"</code></div>
+        <div><em>Does not match:</em> <code>"2"</code>, <code>","</code></div>
+      </div>
+      <div class="try">
+        <em>Try it:</em>
+        <input type="text" value="1,2,3," class="exampleInput" name="paren_pluck">
+        <div class="result"></div>
+      </div>
+    </div>
   </dd>
 
   <dt><code><em>expression</em> *</code></dt>