user management: add note about case-sensitivity for AD realm

user management: add note about case-sensitivity for AD realm

As there its really common to do things case insensitive a docs note
should help users to discover that Proxmox VE allows one to configure
and LDAP/AD realm to also work case insensitive.

While the UI option got added only recently in pve-manager's commit
c34c5418 ("ui: active directory realm: expose case-sensitive option")
mention it already, as there's a good chance that manager gets bumped
sooner than docs.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>