I need to be able to edit nodes from a client outside of Drupal (this cant be implemented as a Drupal module). I'd like to find a module that exposes basic Drupal functionalities as an HTTP API (REST, SOAP, JSON, ...). I found the webservices module, but I cant find any documentation for it.
Do you know where I can find documentation for this module ? Or any other module providing the same kind of functionalities ?
I would suggest checking out the services module. It's in a state of perpetual lack of full releases but it works pretty reliably for the most part. It comes built in with a number of services which hook into the normal Drupal functionality (node, users, menus, etc.)
With it you can either do API key style authentication, or you can just use normal Drupal user accounts. A number of other modules plug into it for different input and output format types.
There are quite a few examples of how to interact with services and there is also there services handbook which has some other reference material as well. I would suggest taking a lookg at it.
If you are ok with XML-RPC, you can use hook_xmlrpc which is part of the Drupal core's API. It allows your module to provide XML-RPC methods without any additional dependencies.