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.
- pipeline:
name: post
- description: This pipeline runs jobs that operate after each change is merged.
+ description: |
+ This pipeline runs jobs that operate after each change is
+ merged. Queue items are identified by the abbreviated hash (git
+ log --format=%h) of the merge commit.
manager: independent
precedence: low
post-review: True
