Update linode-managed.md

[ztemqnoj]

Add verbiage on what needs to be done to allow customers to choose which Linodes fall under Linode Managed.

[wildmanonline]

The Managed service will apply to all Linodes on an account and will be billed at $100/mo per Linode. If this service is not needed for all Linodes, a second account can be created to separate Managed Linodes from non-Managed Linodes. Once the signup is completed for that second account, you can transfer Linodes between accounts by opening up a Support ticket requesting the transfer.

¯_(ツ)_/¯

[ztemqnoj]

Updated verbiage

[nmelehan]

Hi @swarlos518 -

This PR's branch was created from an older version of our master branch which has fallen behind the current version of the site, and so the PR has merge conflicts.

I have rebased and squashed your commits along with those from @kodumbeats, @wildmanonline, and @wireframeskull into a new PR that branches from an up-to-date version of our site, which avoids merge conflicts:

#2220

I'm going to close this current PR, and another member of the docs team will review and merge the new PR.