Request for Documentation: add azure ad notes to ldap docs

[matterdoc]

Mattermost user sven.huester from https://community.mattermost.com has requested the following be documented:

1. You need additionally Azure AD Domain Services for the Azure AD enabled
2. You need to define the security groups you want to sync, or sync all.
3. For the LDAP Group Sync feature, you need to define Security Groups (and sync those over via Azure AD Domain Services) - You cannot use Office 365 groups
4. Azure AD Domain Services only support Secure LDAP (LDAPS) which you can enable under the `Secure LDAP` setting in its config. You may need to allow `Secure LDAP Access over the internet` depending on the network environment. if that's the case, set the security group accordingly to limit access.
5. There is a delay between the sync of Azure AD <-> Azure AD Domain Services, it's not real time. Just be aware of that.
6. For Authentication, you need to Enable Azure AD Domain Services password hash syncronization. Follow their config guide how to do so.

See the original post here.

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[cwarnermm]

@svelle - Does this documentation request continue to be valid? If so, would you be open to creating a PR with these changes?