Clarify docs re TLS support in database filters

[phlax]

There have been various discussions recently about TLS support in database filters

The docs dont provide too much information on this and in the case of mysql it seems contradictory/unclear

It would be good to improve the docs to clarify exactly the status regarding termination and upstream (opportunistic) TLS connections, and perhaps add relevant examples to the sandoxes

[phlax]

some more/less relevant threads

• postgres: support for upstream SSL #23990
• Opportunistic encryption for downstream envoy #7135
• Add support for Envoy's MySQL and Postgres filter types hashicorp/consul#9821