[time] document that integer nanoseconds may be more precise than float

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-01-29.13:02:45.749>
labels = ['3.8', '3.9', '3.10', 'extension-modules', 'type-feature', 'docs']
title = '[time] document that integer nanoseconds may be more precise than float'
updated_at = <Date 2021-03-21.21:08:52.484>
user = 'https://github.com/vxgmichel'

bugs.python.org fields:

activity = <Date 2021-03-21.21:08:52.484>
actor = 'eryksun'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Extension Modules']
creation = <Date 2020-01-29.13:02:45.749>
creator = 'vxgmichel'
dependencies = []
files = ['48880', '48881', '48882', '48883']
hgrepos = []
issue_num = 39484
keywords = []
message_count = 31.0
messages = ['360958', '360980', '361139', '361140', '361141', '361142', '361143', '361144', '361286', '361288', '361292', '361296', '361300', '361301', '361304', '361306', '361308', '361309', '361314', '361316', '361319', '361351', '361650', '361651', '361652', '361653', '361654', '361665', '361666', '361668', '361669']
nosy_count = 10.0
nosy_names = ['lemburg', 'rhettinger', 'mark.dickinson', 'vstinner', 'larry', 'stutzbach', 'docs@python', 'serhiy.storchaka', 'eryksun', 'vxgmichel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39484'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']