Add standard docs text to abstract base classes

src/ol/format/featureformat.js

@@ -6,7 +6,9 @@ goog.require('goog.functions');
 
 /**
  * @classdesc
- * Abstract base class for feature formats.
+ * Abstract base class; normally only used for creating subclasses and not
+ * instantiated in apps.
+ * Base class for feature formats.
  * {ol.format.Feature} subclasses provide the ability to decode and encode
  * {@link ol.Feature} objects from a variety of commonly used geospatial
  * file formats.  See the documentation for each format for more details.

src/ol/format/jsonfeatureformat.js

@@ -10,7 +10,9 @@ goog.require('ol.format.FormatType');
 
 /**
  * @classdesc
- * Abstract base class for JSON feature formats.
+ * Abstract base class; normally only used for creating subclasses and not
+ * instantiated in apps.
+ * Base class for JSON feature formats.
  *
  * @constructor
  * @extends {ol.format.Feature}

src/ol/format/textfeatureformat.js

@@ -8,7 +8,9 @@ goog.require('ol.format.FormatType');
 
 /**
  * @classdesc
- * Abstract base class for text feature formats.
+ * Abstract base class; normally only used for creating subclasses and not
+ * instantiated in apps.
+ * Base class for text feature formats.
  *
  * @constructor
  * @extends {ol.format.Feature}

src/ol/format/xmlfeatureformat.js

@@ -11,7 +11,9 @@ goog.require('ol.xml');
 
 /**
  * @classdesc
- * Abstract base class for XML feature formats.
+ * Abstract base class; normally only used for creating subclasses and not
+ * instantiated in apps.
+ * Base class for XML feature formats.
  *
  * @constructor
  * @extends {ol.format.Feature}