Concept mising in the layout description

Added missing concept description

doc/customize.doc

@@ -198,6 +198,9 @@ The toplevel structure of the file looks as follows:
   <namespace>
     ...
   </namespace>
+  <concept>
+    ...
+  </concept>
   <file>
     ...
   </file>
@@ -291,6 +294,8 @@ pages generated by doxygen:
   documented classes, structs, unions, and interfaces.
 - The \c namespace element represents the layout of all pages generated for
   documented namespaces (and also Java packages).
+- The \c concept element represents the layout of all pages generated for
+  documented concepts.
 - The \c file element represents the layout of all pages generated for
   documented files.
 - The \c group element represents the layout of all pages generated for
@@ -317,6 +322,9 @@ The following generic elements are possible for each page:
       This should not be misinterpreted with the doxygen commands \ref cmdauthor
       "\\author" or \ref cmdauthors "\\authors" that generate an author paragraph
       inside a detailed description.
+</dl>
+The following generic elements are possible for each page except the concept page:
+<dl>
 <dt>\c memberdecl
   <dd>Represents the quick overview of members on a page (member declarations).
       This elements has child elements per type of member list.
@@ -348,6 +356,15 @@ The class page has the following specific elements:
       extracted.
 </dl>
 
+The concept page has the following specific elements:
+<dl>
+<dt>\c includes
+  <dd>Represents the include file needed to obtain the definition for
+      this class.
+<dt>\c definition
+  <dd>Represents the definition of the concept
+</dl>
+
 The file page has the following specific elements:
 <dl>
 <dt>\c includes