[doc] move server db info to reference section

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+Server-side databases
+=====================
+
+Soledad Server works with one database per user and one shared database in
+which user's encrypted secrets might be stored.
+
+User databases
+--------------
+
+User databases in the server are named 'user-<uuid>' and Soledad Client may
+perform synchronization between its local replicas and the user's database in
+the server. Authorization for creating, updating, deleting and retrieving
+information about the user database as well as performing synchronization is
+handled by the `leap.soledad.server.auth` module.
+
+Shared database
+---------------
+
+Each user may store password-encrypted recovery data in the shared database.
+
+Recovery documents are stored in the database without any information that
+may identify the user. In order to achieve this, the doc_id of recovery
+documents are obtained as a hash of the user's uid and the user's password.
+User's must have a valid token to interact with recovery documents, but the
+server does not perform further authentication because it has no way to know
+which recovery document belongs to each user.
+
+This has some implications:
+
+  * The security of the recovery document doc_id, and thus of access to the
+    recovery document (encrypted) content, as well as tampering with the
+    stored data, all rely on the difficulty of obtaining the user's password
+    (supposing the user's uid is somewhat public) and the security of the hash
+    function used to calculate the doc_id.
+
+  * The security of the content of a recovery document relies on the
+    difficulty of obtaining the user's password.
+
+  * If the user looses his/her password, he/she will not be able to obtain the
+    recovery document.
+
+  * Because of the above, it is recommended that recovery documents expire
+    (not implemented yet) to prevent excess storage.
+
+The authorization for creating, updating, deleting and retrieving recovery
+documents on the shared database is handled by `leap.soledad.server.auth`
+module.
+