custom data directory

The documentation says this:
`postgresql_pgdata: Only set this if you have changed the $PGDATA directory from the package default. Note this does not configure PostgreSQL to actually use a different directory, you will need to do that yourself, it just allows the role to properly locate the directory.
`
So how would one go about initializing a pg cluster with a custom data directory?

Start with the role documentation describing postgresql_pgdata and the PostgreSQL PGDATA setting. Determine the supported steps for initializing a cluster in a custom data directory, then update the documentation so the configuration and initialization procedure are explicit.

Written by the indexing model from the issue text.