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/utils/checkablemessagebox.cpp

@@ -38,7 +38,8 @@
 /*!
     \class Utils::CheckableMessageBox
 
-     \brief A messagebox suitable for questions with a
+    \brief The CheckableMessageBox class implements a message box suitable for
+    questions with a
      "Do not ask me again" checkbox.
 
     Emulates the QMessageBox API with