Improvements in API documentation

[jnehring]

This issue is a collection of suggestions how to improve the API documentation.

• Remove dublicate parameters like "informat" and "Content-Type" Remove Content-Type header and Accept header from documentation #151
• @andish s suggestions in 151, especially reordering the parameters by "required parameters" on top and then "expert parameters" below.
• The suggestion by @Xfran to get inspiration from http://docs.aylien.com/docs/entities
• Add a general text to each e-Service Add general text to each e-Service #11
• Make general information collapsible Make general information collapsible #37