add docs for connecting via sockets

docs/core/connectors/mysql.md

@@ -53,4 +53,5 @@ See the [official documentation](https://dev.mysql.com/doc/refman/8.0/en/connect
 - `user`: The database user, e.g. `admin`.
 - `password`: The password for the database user.
 - `ssl`: Whether or not your database server uses SSL.
-- `connection_limit` (coming soon): The connection limit specifies the maximum number of simultaneous connections that Prisma might have open to your database. **Default**: `1`.
+- `connection_limit`: The connection limit specifies the maximum number of simultaneous connections that Prisma might have open to your database.
+- `socket`: If you want to connect via sockets, this parameter must specified a file path to the socket you want to use, e.g.: `/var/run/mysqld/mysqld.sock`. (Learn more in this [GitHub issue](https://github.com/prisma/prisma2/issues/525)).

docs/core/connectors/postgresql.md

@@ -87,3 +87,9 @@ You can add various parameters to the connection string if your database server
 - `sslaccept=(strict|accept_invalid_certs)`: 
   - `strict` (default): Any missing value in the certificate will lead to an error. For Google Cloud, especially if the database doesn't have a domain name, the certificate might miss the domain/IP address, causing an error when connecting.
   - `accept_invalid_certs`: Bypass this check. Be aware of the security consequences of this setting.
+
+### Connecting via sockets
+
+To connect to your PostgreSQL database via sockets, you must add a `host` field as a _query parameter_ to the connection string (instead of setting it as the `host` part of the URI). The value of this parameter then must point to the directory that contains the socket, e.g.: `postgresql://user:password@/database?host=/var/run/postgresql/`. 
+
+Learn more in this [GitHub issue](https://github.com/prisma/prisma2/issues/525).