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/coreplugin/textdocument.cpp

@@ -35,7 +35,8 @@
 
 /*!
     \class Core::TextFile
-    \brief Base class for text files with encoding helpers.
+    \brief The TextFile class is a base class for text files with encoding
+    helpers.
 
     Stores the format obtained from read operations and uses that when writing
     out files, thus ensuring that CRLF/encodings are preserved.