[API] Add user API docs

[nadouani]

Add documentation of the following APIs:

• [x] Create user
• [x] Update user
• [x] List/search users
• [x] Set user password
• [x] Generate API key for a user
• [x] Revoke API key of a user
• [x] Get API key of a user
• [x] Lock user
• [x] Delete user
• [x] Review

[To-om]

The documentation doesn't contains information on how the user avatar is retrieved nor updated.