[doc] improve documentation on IBlobsBackend

docs/api/blobs-server-backend.rst

-Blobs Server-side Data Backend API
-==================================
+Blobs Server-side Backend Interface
+===================================
 
-Blobs Backend Interface
------------------------
+The :ref:`blobs` functionality has the purpose of adding to Soledad the
+capacity of handling binary objects efficiently. In this case, efficiency means
+low use of resources (memory, cpu, and time) and storage scalability. The
+choice of which data storage backend to use for Blobs affects each of these
+properties in different ways.
+
+The :ref:`blobs-backend-interface` is provided so that Soledad Server is
+agnostic of which data backend is used for Blobs storage. In order to add a new
+backend, one needs to:
+
+* implement a backend according to the :ref:`IBlobsBackend <blobs-backend-interface>` interface,
+* register the new handler in the Twisted-based ``BlobsResource``, and
+* instantiate the Twisted-based ``BlobsResource`` using the new handler.
+
+.. _blobs-backend-interface:
+
+``IBlobsBackend`` Interface
+---------------------------
 
 .. autoclass:: leap.soledad.server.interfaces.IBlobsBackend
    :members:
    :undoc-members:
-   :special-members: __init__
    :show-inheritance: