Reference TOC: expand 'Atoms' and 'Primaries'

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-04-04.18:31:12.645>
labels = ['type-feature', '3.7', 'docs']
title = "Reference TOC: expand 'Atoms' and 'Primaries'"
updated_at = <Date 2017-04-04.18:45:31.139>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2017-04-04.18:45:31.139>
actor = 'terry.reedy'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2017-04-04.18:31:12.645>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 29983
keywords = []
message_count = 2.0
messages = ['291131', '291133']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'docs@python']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue29983'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7']