Clarify what the PG-prefixed variables are for

roles/custom/matrix-synapse-auto-compressor/templates/env.j2

@@ -1,4 +1,6 @@
 POSTGRES_LOCATION={{ matrix_synapse_auto_compressor_environment_variable_postgres_location }}
+
+# PG-prefixed variables below are for the matrix-synapse-auto-compressor-fix.sh script
 PGHOST={{ matrix_synapse_auto_compressor_database_hostname }}
 PGDATABASE={{ matrix_synapse_auto_compressor_database_name }}
 PGUSER={{ matrix_synapse_auto_compressor_database_username }}