Improve documentation about converting datetime.timedelta to scalars

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-02-27.14:54:37.765>
labels = ['3.8', '3.7', 'library', 'docs']
title = 'Improve documentation about converting datetime.timedelta to scalars'
updated_at = <Date 2019-03-16.05:34:12.160>
user = 'https://github.com/pganssle'

bugs.python.org fields:

activity = <Date 2019-03-16.05:34:12.160>
actor = 'ncoghlan'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Library (Lib)']
creation = <Date 2019-02-27.14:54:37.765>
creator = 'p-ganssle'
dependencies = []
files = []
hgrepos = []
issue_num = 36138
keywords = ['patch']
message_count = 5.0
messages = ['336765', '336980', '338052', '338053', '338054']
nosy_count = 7.0
nosy_names = ['fdrake', 'ncoghlan', 'belopolsky', 'docs@python', 'p-ganssle', 'miss-islington', 'Yasser Alshalaan']
pr_nums = ['12137', '12362']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue36138'
versions = ['Python 3.7', 'Python 3.8']