Fixed documentation described in issue #232

The issue was described as: 

Positions of incorrect mention of `RequestURITooLarge`: 
* https://github.com/mitsuhiko/werkzeug/blob/master/werkzeug/formparser.py#L78
* https://github.com/mitsuhiko/werkzeug/blob/master/werkzeug/formparser.py#L121

Positions where `RequestEntityTooLarge` is raised from conditions relating to `max_form_memory_size`: 
* https://github.com/mitsuhiko/werkzeug/blob/master/werkzeug/formparser.py#L207
* https://github.com/mitsuhiko/werkzeug/blob/master/werkzeug/formparser.py#L339

I rebuilt the documentation and confirmed that the new documentation is correct.