[doc] proofreading the guides, refactoring the reference.

docs/guide/future/services.rst

@@ -0,0 +1,56 @@
+Services
+========
+
+.. warning::
+
+   This is a "future" document, that does not exist, it's only kept here not to lose the data until we organize better
+   documentation versioning.
+
+Future and proposals
+::::::::::::::::::::
+
+This is a first implementation and it will evolve. Base concepts will stay the same though.
+
+May or may not happen, depending on discussions.
+
+* Singleton or prototype based injection (to use spring terminology, see
+  https://www.tutorialspoint.com/spring/spring_bean_scopes.htm), allowing smart factory usage and efficient sharing of
+  resources.
+* Lazily resolved parameters, eventually overriden by command line or environment, so you can for example override the
+  database DSN or target filesystem on command line (or with shell environment vars).
+* Pool based locks that ensure that only one (or n) transformations are using a given service at the same time.
+* Simple config implementation, using a python file for config (ex: bonobo run ... --services=services_prod.py).
+* Default configuration for services, using an optional callable (`def get_services(args): ...`). Maybe tie default
+  configuration to graph, but not really a fan because this is unrelated to graph logic.
+* Default implementation for a service in a transformation or in the descriptor. Maybe not a good idea, because it
+  tends to push forward multiple instances of the same thing, but maybe...
+
+  A few ideas on how it can be implemented, from the user perspective.
+
+  .. code-block:: python
+
+      # using call
+      http = Service('http.client')(requests)
+
+      # using more explicit call
+      http = Service('http.client').set_default_impl(requests)
+
+      # using a decorator
+      @Service('http.client')
+      def http(self, services):
+          import requests
+          return requests
+
+      # as a default in a subclass of Service
+      class HttpService(Service):
+          def get_default_impl(self, services):
+              import requests
+              return requests
+
+      # ... then use it as another service
+      http = HttpService('http.client')
+
+
+This is under development, let us know what you think (slack may be a good place for this).
+The basics already work, and you can try it.
+