* Mention PostgreSQL in sql_database_instance docs
As mentioned in hashicorp/terraform#12617 the
google_sql_database_instance resource already supports `POSTGRES_9_6` as
`database_version`, it is just undocumented so far.
* Make naming of google_sql_database consistent
Looks like this is a relict from some copy & paste from
`google_storage_*` while writing the docs for `google_sql_database`.
* Note that Postgres support is still BETA