Document ability to define SSL context via Sanic app config

Author: illia-v

docs/reference/extensions/sanic.rst

@@ -25,6 +25,7 @@ Supported configurations:
 - DB_ECHO
 - DB_POOL_MIN_SIZE
 - DB_POOL_MAX_SIZE
+- DB_SSL
 - DB_USE_CONNECTION_FOR_REQUEST
 - DB_KWARGS
 
@@ -80,6 +81,7 @@ To integrate with Sanic, a few configurations needs to be set in
 - DB_ECHO: if not set, ``False``
 - DB_POOL_MIN_SIZE: if not set, 5
 - DB_POOL_MAX_SIZE: if not set, 10
+- DB_SSL: if not set, ``None``
 - DB_KWARGS; if not set, empty dictionary
 
 An example: