Docs: update docs about ipaplatform to match reality
Apparently, the docs were not updated when ipapython/platform was moved
to ipaplatform module and internals have changed.
https://fedorahosted.org/freeipa/ticket/6418
Reviewed-By: Lukas Slebodnik <lslebodn@redhat.com>
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>