Clarify DISALLOW_ADD_USER for DO

DISALLOW_ADD_USER is applied for organization-owned devices, even if not provisioned with a managed profile, so the text should be updated accordingly.


Bug: 394910130
Change-Id: I3a8659715506d7dd199b4651d5e143d99e7e7770
Test: N/A (javadoc only)
Flag: EXEMPT doc only