New doc: clickdata in Formrequest.from_response

Documentation about:
 * clickdata parameter in Formrequest.from_response
 * nr attribute in clickdata dict
 * default behaviour when clickdata is None
FormRequest doc improvements

Clickdata doc enhancements:
 * Fix xml attributes mention
 * nr is 0-indexed reference
Merge pull request #645 from Curita/formrequest-clickdata-doc

Documentation for FormRequest.from_response clickdata parameter