Update ENVIRONMENT.rst to reflect nature of PGHOME, PGROOT and PGDATA

[robinp]

See https://github.com/zalando/spilo/blob/master/postgres-appliance/Dockerfile#L100-L108 and #619 (comment)

[zalando-robot]

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[robinp]

Note: mildly unrelated, but PG_CONNECT_ADDRESS is missing from the enumeration completely. It is the "advertised address" towards peer Patroni-s/Spilo-s about where to connect to the primary for replication.