update docs for AppData
Application-Defined Data (AppData)
==================================
+The application-defined data feature is not very well documented in the DXF
+reference, so usage as custom data store is not recommended. AutoCAD
+uses these feature to store the handle to the extension dictionary
+(:class:`~ezdxf.entities.xdict.ExtensionDict`) of a DXF entity and the handles
+to the persistent reactors (:class:`~ezdxf.entities.appdata.Reactors`) of a
+DXF entity.
+Use the high level methods of :class:`~ezdxf.entities.DXFEntity` to manage
+application-defined data tags.
+
+- :meth:`~ezdxf.entities.DXFEntity.has_app_data`
+- :meth:`~ezdxf.entities.DXFEntity.get_app_data`
+- :meth:`~ezdxf.entities.DXFEntity.set_app_data`
+- :meth:`~ezdxf.entities.DXFEntity.discard_app_data`
+
+.. hint::
+
+ Ezdxf uses special classes to manage the extension dictionary and the
+ reactor handles. These features cannot be accessed by the methods above.
+
+Set application-defined data::
+
+ entity.set_app_data("YOURAPPID", [(1, "DataString")]))
+
+Setting the content tags can contain the opening structure tag
+(102, "{YOURAPPID") and the closing tag (102, "}"), but doesn't have to.
+The returned :class:`~ezdxf.lldxf.tags.Tags` objects does not contain these
+structure tags. Which tags are valid for application-defined data is not
+documented.
+
+The AppID has to have an entry in the AppID table.
+
+Get application-defined data::
+
+ if entity.has_app_data("YOURAPPID"):
+ tags = entity.get_app_data("YOURAPPID")
+
+ # tags content is [DXFTag(1, 'DataString')]
.. seealso::
