story#5589: read a meaningful api documentation

+ Token resource returns 500 errors on exceptions instead of 400 Bad
Request
+ POST /tokens needs {username,password} in body instead of {authentication}
+ API explorer displays the returned type for the following routes:
  + POST /tokens
  + GET /projects/:id
  + GET /trackers/:id
  + GET /artifacts/:id
  + GET /milestones/:id
  + GET /*/:id/milestones
  + GET /projects/:id/plannings
  + GET /projects/:id/trackers
  + GET /milestones/:id/content
  + GET /milestones/:id/backlog

Change-Id: I38279ed8928f1d82cddf6e1f81cf36278d9d39c1
Merge remote-tracking branch 'gerrit/5589_read_a_meaningful_api_documentation' into stable

* gerrit/5589_read_a_meaningful_api_documentation:
  story#5589: read a meaningful api documentation

Change-Id: Ib19e3bef8f1a7e49a44dde894a1ce7f6443e9424
This is Tuleap 6.7.99.23

Change-Id: Iffc394d55dc0e516f066e0dfb0ccd236208799a4