Add docs on how to properly change the `postgres` password.

[davissp14]

If a user changes the postgres password without updating the associated OPERATOR_PASSWORD secret, it will impact their ability to boot on their next restart.

[davissp14]

Related: #131

[guillaumervls]

I'm not seeing lots of users being bitten by this in the forum. Adding the "not now" flag.

Also noting that we could also documents other secrets' meaning and purposes.