networking: document disabling dhcp on every interface behavior

[dustymabe]

NM by default will try to DHCP on every interface that's not explicitly configured. Let's document how to disable that behavior because sometimes bringing up interfaces that aren't desired to be brought up has unintended consequences. Example:

storage:                                                                                                                                                                                                                                     
  files:                                                                                                                                                                                                                                                                                                                                                                                                                                             
    - path: /etc/NetworkManager/conf.d/10-disable-unconfigured-nics.conf                                                                                                                                                                     
      mode: 0644                                                                                                                                                                                                                             
      contents:                                                                                                                                                                                                                              
        inline: |                                                                                                                                                                                                                            
          [main]                                                                                                                                                                                                                             
          no-auto-default=*

As part of this we should link to the NM documentation that explains what this does in more detail.