Improve docs for await expression

GitHub fields:

assignee = 'https://github.com/aeros'
closed_at = None
created_at = <Date 2019-12-18.03:47:41.745>
labels = ['3.9', 'type-feature', '3.8', 'docs', 'expert-asyncio']
title = 'Improve docs for await expression'
updated_at = <Date 2020-06-29.15:39:27.048>
user = 'https://github.com/aeros'

bugs.python.org fields:

activity = <Date 2020-06-29.15:39:27.048>
actor = 'wwallace'
assignee = 'aeros'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'asyncio']
creation = <Date 2019-12-18.03:47:41.745>
creator = 'aeros'
dependencies = []
files = []
hgrepos = []
issue_num = 39085
keywords = []
message_count = 4.0
messages = ['358610', '358621', '358660', '372594']
nosy_count = 5.0
nosy_names = ['njs', 'asvetlov', 'yselivanov', 'aeros', 'wwallace']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39085'
versions = ['Python 3.8', 'Python 3.9']