Document 'post' job names
Document 'post' job names

Some may consider it confusing that most jobs on zuul.openstack.org are
named after their review numbers, but `post` jobs aren't.  This change
adds words to the description of the post queue indicating that jobs are
named after the abbreviated version of the commit hash of the merge.

Change-Id: I8a304b302d13468f6cbb73edadf60c6667ec32e3

openstack-infra/project-config,openstack-infra/project-config