Docs unclear on `relayhost`'s `direct` value

[fayeg21]

Affected Puppet, Ruby, OS and module versions/distributions

• Puppet:
• Ruby:
• Distribution:
• Module version: v3.0.0

How to reproduce (e.g Puppet code you use)

What are you seeing

In https://forge.puppet.com/modules/puppet/postfix/readme#relayhost it's not mentioned that when "mta => true" then the special value of "direct" can be used for relayhost

Without knowing this one gets this cryptic error message:

Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Function Call, pick(): must receive at least one non empty value (file: /etc/puppetlabs/code/modules/postfix/manifests/mta.pp, line: 43, column: 17)

What behaviour did you expect instead

1. A better error message
2. The 'direct' setting added to the 'relayhost' doc in the README.

Output log

Any additional information you'd like to impart

[ekohl]

Isn't this a duplicate of #295?