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/plugins/vcsbase/cleandialog.cpp

@@ -144,7 +144,8 @@ CleanDialogPrivate::CleanDialogPrivate() :
 /*!
     \class VcsBase::CleanDialog
 
-    \brief File selector dialog for files not under version control.
+    \brief The CleanDialog class provides a file selector dialog for files not
+    under version control.
 
     Completely clean a directory under version control
     from all files that are not under version control based on a list