docs-xml: change the default for "map untrusted to domain" to "auto"

docs-xml: change the default for "map untrusted to domain" to "auto"

This makes the behaviour much more robust, particularly with forest child
domains over one-way forest trusts.

Sadly we don't support this kind of setup with our current ADDC, so
there's no way to have automated tests for this behaviour, but
at least we know it doesn't break any existing tests.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=8630

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>