IDLE: Add docstrings and tests for editor.py reload functions

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2017-09-20.13:06:20.730>
labels = ['expert-IDLE', 'type-feature', '3.7']
title = 'IDLE: Add docstrings and tests for editor.py reload functions'
updated_at = <Date 2018-05-23.04:43:16.595>
user = 'https://github.com/csabella'

bugs.python.org fields:

activity = <Date 2018-05-23.04:43:16.595>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2017-09-20.13:06:20.730>
creator = 'cheryl.sabella'
dependencies = []
files = []
hgrepos = []
issue_num = 31529
keywords = ['patch']
message_count = 4.0
messages = ['302609', '302704', '311623', '317352']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'cheryl.sabella']
pr_nums = ['3669']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31529'
versions = ['Python 3.6', 'Python 3.7']