JBTM-1429 Document the PartcipantCompletion race condition
Merge branch '4.17-JBTM-1429_Document_the_PartcipantCompletion_race_condition' of git://github.com/paulrobinson/narayana into paulrobinson-4.17-JBTM-1429_Document_the_PartcipantCompletion_race_condition