Integrate two leftover descriptive chapters from the old REST API reference:

• https://github.com/ezsystems/ezpublish-kernel/blob/v8.0.0-beta5/doc/specifications/rest/REST-API-V2.rst#general-considerations
• https://github.com/ezsystems/ezpublish-kernel/blob/v8.0.0-beta5/doc/specifications/rest/REST-API-V2.rst#authentication

into the https://doc.ezplatform.com/en/master/api/rest_api_guide/ developer doc section.