Doc: use standard wording in \brief commands

QDoc does some magic with the \class and \namespaces
and \brief commands, so the following wording must be used:
"The xxx class yyy ..."

Change-Id: Id231f30e8464898b776888d5423523de404aae34
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/libs/cplusplus/PreprocessorClient.cpp

@@ -33,7 +33,8 @@ using namespace CPlusPlus;
 
 /*!
     \class Client
-    \brief A notification interface for for C++ preprocessor.
+    \brief The Client class implements a notification interface for the
+    C++ preprocessor.
 */
 
 /*!