Database restoration documentation could use some clarification

[whi-tw]

As per support ticket #128546, "It might be an idea to reword the documentation slightly; as it stands there's a couple of places which a bit definitive on the fact that only the very latest snapshot can be restored manually"

Specifically:

• https://github.com/alphagov/paas-tech-docs/blame/master/source/documentation/deploying_services/postgresql.md#L565
• https://github.com/alphagov/paas-tech-docs/blame/master/source/documentation/deploying_services/postgresql.md#L577
• https://github.com/alphagov/paas-tech-docs/blame/master/source/documentation/deploying_services/postgresql.md#L625