Document manage_symlink_source properity for systemd_unit

User Story

As a <role: Developer, SysAdmin, etc.>, I will use this work when I want to ... Manage systemd unit properly.

Description

<! Briefly describe the issue > The documentation is missing manage_symlink_source property. I saw a warning about this property when running in test kitchen with chef-client 14.2

Chef Version

<! Tell us which version of chefclient, chefserver, or ChefDK you need docs for) > chef-client 14.2

Platform Version

<! Tell us which Operating System distribution and version you're running Chef on. >

Definition of Done

manage_symlink_source is list in properties for systemd_unit resource.

Demo Script / Repro Steps

So apparently I can't set it on the resource and yet I see this warning: [2018-08-07T14:34:41+00:00] WARN: Disable this warning by setting manage_symlink_source true on the resource [2018-08-07T14:34:41+00:00] WARN: In a future Chef release, 'manage_symlink_source' will not be enabled by default