IDLE: Clarify some completion details in doc

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2020-07-11.07:12:43.928>
labels = ['expert-IDLE', 'type-feature', '3.10']
title = 'IDLE: Clarify some completion details in doc'
updated_at = <Date 2020-07-16.04:33:16.329>
user = 'https://github.com/Alex-Python-Programmer'

bugs.python.org fields:

activity = <Date 2020-07-16.04:33:16.329>
actor = 'Alex-Python-Programmer'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2020-07-11.07:12:43.928>
creator = 'Alex-Python-Programmer'
dependencies = []
files = []
hgrepos = []
issue_num = 41278
keywords = []
message_count = 8.0
messages = ['373518', '373524', '373625', '373635', '373638', '373662', '373674', '373740']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'Alex-Python-Programmer', 'epaine']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41278'
versions = ['Python 3.10']