clearly document the effect of empty includesfile attribute

Bugzilla Issue 26814

manual/Types/dirset.html

@@ -61,7 +61,10 @@ equivalent to an <code>&lt;and&gt;</code> selector container.</p>
   <tr>
     <td valign="top">includesfile</td>
     <td valign="top">The name of a file; each line of this file is
-      taken to be an include pattern.</td>
+      taken to be an include pattern.
+      <b>Note:</b> if the file is empty and there are no other
+      patterns defined for the fileset, all directories will be included.
+    </td>
     <td valign="top" align="center">No</td>
   </tr>
   <tr>

manual/Types/fileset.html

@@ -71,7 +71,10 @@ equivalent to an <code>&lt;and&gt;</code> selector container.</p>
   <tr>
     <td valign="top">includesfile</td>
     <td valign="top">the name of a file; each line of this file is
-      taken to be an include pattern.</td>
+      taken to be an include pattern.<br/>
+      <b>Note:</b> if the file is empty and there are no other
+      patterns defined for the fileset, all files will be included.
+    </td>
     <td valign="top" align="center">No</td>
   </tr>
   <tr>

manual/Types/patternset.html

@@ -50,7 +50,10 @@ or the following attributes.</p>
     <td valign="top">includesfile</td>
     <td valign="top">the name of a file; each line of this file is
       taken to be an include pattern. You can specify more than one
-      include file by using a nested includesfile elements.</td>
+      include file by using a nested includesfile elements.
+      <b>Note:</b> if the file is empty and there are no other
+      patterns defined for the fileset, all files will be included.
+    </td>
   </tr>
   <tr>
     <td valign="top">excludes</td>