Documentation for ol.format.Feature subclasses

This PR copies documentation from `ol.format.Feature` into the subclasses that are actually exported to generate documentation for these classes.

In order to document functions inherited from base classes that are not redefined in the exported class, we need to include the function name in the source file and use the `@function` keyword to tell jsdoc that we are documenting a function.

Some of the exports have been adjusted based on my understanding of what is actually available from the public API in each of the format classes.  Of particular note, I removed `readGeometry` from the KML format as it seems not to be defined in the KML format and thus inherits the abstract method implementation from the XML format.  If any choices seem inappropriate, please let me know.