Editors: Add at least some short documentation.

Change-Id: I7fc7946405d3cbb1af0cf29a9d328d857eb8304a
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/texteditor/basetexteditor.cpp

@@ -104,6 +104,14 @@
     \internal
 */
 
+/*!
+    \class TextEditor::BaseTextEditor
+    \brief The BaseTextEditor class is a base class for QPlainTextEdit-based text editors.
+
+    The corresponding document base class is BaseTextDocument, the corresponding widget base class
+    is BaseTextEditorWidget.
+*/
+
 using namespace Core;
 using namespace Utils;