Update Notification interface to match correct API

[ScreamZ]

Improve document payload according to real documentation

https://docs.kuzzle.io/core/2/api/payloads/notifications/#document-notification

Can you ping me on release @rolljee @etrousset ? Thanks :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.86%. Comparing base (dbe0603) to head (fbb6d69).
Report is 26 commits behind head on 7-dev.

Additional details and impacted files

@@           Coverage Diff           @@
##            7-dev     #739   +/-   ##
=======================================
  Coverage   82.86%   82.86%           
=======================================
  Files          46       46           
  Lines        2119     2119           
  Branches      410      410           
=======================================
  Hits         1756     1756           
  Misses        282      282           
  Partials       81       81           

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