✍️ Improve Postgres docs on password/auth environment vars

[babakks]

Current documentations on PostgreSQL image, are a bit unclear about the required, non-empty POSTGRES_PASSWORD environment variable.

Now, more explanations are added and clearer emphasis on important notes are inserted.

[riconnon]

This tripped me up too when upgrading from 9.6.16-alpine to 9.6.17-alpine I did not expect to find a significant change in behaviour.
This docs change would definitely help.

[yosifkit]

Rebased and moved a bit around. Only document POSTGRES_PASSWORD being not required in the POSTGRES_HOST_AUTH_METHOD section. Also added some PGDATA improvements.