PCI: rpaphp: Document find_php_slot()

Document some of the interaction between dlpar and hotplug.
viz, the a dlpar remove of a htoplug slot uses hotplug to remove it.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Cc: John Rose <johnrose@austin.ibm.com>
Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>