Add note about Postgres requiring user passwords

[s3bw]

As it's been noted this field is required on instances of Postgres: #4123 (comment)

This was previously changed since some instances don't require a password, and the documentation had to display the "lowest common level of validation".

the issue here is that Postgres instances require a password and will fail without one but MySql instances don't

Last updated: #1056

You'll notice in the debug output provided by the user in issue #4123 shows:

Missing user password for PostgreSQL instance

I think this PR might close issue 4123?

[rileykarson]

LGTM. I'll upstream this so it gets applied to google-beta as well, then merge this PR.

[ghost]

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