docs: suggest target_session_attrs=read-write when using HA and psql

[desimone]

    I deployed [git-3b2cc672](https://hub.docker.com/layers/pomerium/pomerium/git-3b2cc672/images/sha256-45f01a6e2777a902c78aba439185e0ffcb30d12ff0e058e4ddf1b38b58a2622d?context=explore) today.  Things looked good - all 3 PSQL nodes came online (using basically the config I posted), pomerium connected to the PSQL backend, and it served traffic.  I successfully took down the primary PSQL node and the databroker survived, switching over to the new primary.

## WORKING
# Use HA PSQL databroker backend
databroker_storage_type: postgres
databroker_storage_connection_string: postgres:https://pomerium:pomerium@pg-0,pg-1,pg-2/pomerium?sslmode=disable&target_session_attrs=read-write

===

However, a word of caution! target_session_attrs=read-write is absolutely critical. Without it libpq correctly switches to a second node, but has a chance to end up on a read-only replica. When I tested this, the constant write attempts eventually took down repmgr and even the entire PSQL container.

{"level":"info","syncer_id":"databroker","syncer_type":"type.googleapis.com/pomerium.config.Config","time":"2022-10-04T13:08:04-05:00","message":"initial sync"}
{"level":"info","type":"type.googleapis.com/pomerium.config.Config","time":"2022-10-04T13:08:04-05:00","message":"sync latest"}
{"level":"error","syncer_id":"databroker","syncer_type":"type.googleapis.com/pomerium.config.Config","error":"rpc error: code = Unknown desc = ERROR: cannot execute CREATE SCHEMA in a read-only transaction (SQLSTATE 

## BROKEN
# Use HA PSQL databroker backend
databroker_storage_type: postgres
databroker_storage_connection_string: postgres:https://customuser:custompassword@pg-0,pg-1,pg-2/pomerium?sslmode=disable

I'm not sure how the databroker works under the hood; perhaps on large installs it can make use of read-only replicas for load balancing? However, attempting to write on a read-only connection makes no sense to me. Arguably this behavior should be changed, or at the least the docs can be clarified in the docs to stress using target_session_attrs=read-write when using HA.

Originally posted by @alexrudd2 in pomerium/pomerium#3634 (comment)