Documentation for setting PLAIN authentication with python SDK

Currently we don't have the documentation for setting plain authentication with Python SDK.

The way to do it is to set it in the connection string like so:

cluster = Cluster("couchbases://10.143.205.101?sasl_mech_force=PLAIN&certpath=cert.ca",ClusterOptions(PasswordAuthenticator("einstein","password")))

We need to document this like we did for Java here:

https://docs.couchbase.com/java-sdk/current/howtos/sdk-authentication.html