Doc: move Class View plugin docs to .cpp files.

QDoc does not find docs in the .h files.
Use \brief only for namespaces, classes, enums, and properties.
Use standard wording for \brief and \fn.
Use \a and \c according to guidelines.

Fix grammar and style.

Change-Id: Ib685a03c97ef38661ecc156f61d70085514357fc
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/classview/classviewplugin.cpp

@@ -38,6 +38,14 @@ namespace Internal {
 
 ///////////////////////////////// Plugin //////////////////////////////////
 
+/*!
+    \class Plugin
+    \brief The Plugin class is the base class for the Class View plugin.
+
+    The Class View shows the namespace and class hierarchy of the currently open
+    projects in the sidebar.
+*/
+
 bool Plugin::initialize(const QStringList &arguments, QString *errorMessage)
 {
     Q_UNUSED(arguments)