Document the differences between str.isdigit, isdecimal and isnumeric

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-02-24.09:02:22.132>
labels = ['type-feature', 'docs']
title = 'Document the differences between str.isdigit, isdecimal and isnumeric'
updated_at = <Date 2019-02-24.13:32:58.519>
user = 'https://bugs.python.org/StyXman'

bugs.python.org fields:

activity = <Date 2019-02-24.13:32:58.519>
actor = 'steven.daprano'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2019-02-24.09:02:22.132>
creator = 'StyXman'
dependencies = []
files = []
hgrepos = []
issue_num = 36100
keywords = []
message_count = 12.0
messages = ['336451', '336453', '336454', '336455', '336456', '336459', '336460', '336461', '336462', '336464', '336466', '336467']
nosy_count = 5.0
nosy_names = ['mark.dickinson', 'StyXman', 'steven.daprano', 'docs@python', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36100'
versions = []