docs: document the behaviour of using special values of timestamp with subscriptions

# Description

Postgres supports certain special timestamp with zone values as mentioned [here](https://www.postgresql.org/docs/current/datatype-datetime.html#DATATYPE-DATETIME-SPECIAL-VALUES). These values can be used with graphql-engine but sometimes they can have unexpected behavior as mentioned [here](https://github.com/hasura/graphql-engine/issues/8553).

We should document this in our docs that using the special postgres timestamp values can have unexpected behaviour and it is always better to use a hardcoded timestamp value.