Mutating a list while iterating: clarify the docs

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-02-04.19:27:00.850>
labels = ['type-feature', 'docs']
title = 'Mutating a list while iterating:  clarify the docs'
updated_at = <Date 2018-02-11.03:02:24.225>
user = 'https://github.com/tim-one'

bugs.python.org fields:

activity = <Date 2018-02-11.03:02:24.225>
actor = 'tim.peters'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2018-02-04.19:27:00.850>
creator = 'tim.peters'
dependencies = []
files = []
hgrepos = []
issue_num = 32767
keywords = []
message_count = 6.0
messages = ['311614', '311700', '311735', '311791', '311932', '311977']
nosy_count = 6.0
nosy_names = ['tim.peters', 'rhettinger', 'terry.reedy', 'docs@python', 'Stefan Pochmann', 'xgdomingo']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32767'
versions = ['Python 3.5']