Rewrite the transit policy section
Rewrite the transit policy section

Expand the transit policy section considerably, with additional
examples and explanation of the examples.  Separate allowing
cross-realm transits from configuring clients to do cross-realm
transits.  Add a separate example section for an Active Directory
forest.

Signed-off-by: Love Hornquist Astrand <c5e2f0824a6b8bf7ea1d693b4c00a7e90e09ef7a@h5l.org>

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