IMO learn more should go in long summary.

e.g. I would not want to show that level of details in this command:

 D:\>az devops security permission -h

Group
    az devops security permission : Manage security permissions.
        This command group is in preview. It may be changed/removed in a future release.
Subgroups:
    namespace : Manage security namespaces.

Commands:
    list      : List tokens for given user/group and namespace.
    reset     : Reset permission for given permission bit(s).
    reset-all : Clear all permissions of this token for a user/group.
    show      : Show permissions for given token, namespace and user/group.
    update    : Assign allow or deny permission to given user/group.

Good catch! I agree, it will make more sense if we include the link in the long summary.
@geverghe what do you think ?

I am not sure of how the long summary appears. @atbagga / @ishitam8 any prior examples where long summary has been used to get a sense of how it would look?

Ok got it; makes perfect sense. Including the reference I got from cognitive services that follows the same structure:

long-summary: This article lists the Azure CLI commands for Azure Cognitive Services account and subscription management only. Refer to the documentation at https://docs.microsoft.com/azure/cognitive-services/ for individual services to learn how to use the APIs and supported SDKs.

This is how it looks if added in long-summary with example

Raised a fix in Knack for the preview message text to start in new line.
microsoft/knack#167