Documentation: Clarify role of callback with map_async

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-12-29.21:57:31.364>
labels = ['type-bug', 'docs']
title = 'Documentation: Clarify role of callback with map_async'
updated_at = <Date 2020-09-18.16:03:49.957>
user = 'https://bugs.python.org/JoseMiguelColella'

bugs.python.org fields:

activity = <Date 2020-09-18.16:03:49.957>
actor = 'iritkatriel'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2016-12-29.21:57:31.364>
creator = 'Jose Miguel Colella'
dependencies = []
files = ['46084']
hgrepos = []
issue_num = 29108
keywords = []
message_count = 4.0
messages = ['284295', '284296', '284305', '284320']
nosy_count = 3.0
nosy_names = ['docs@python', 'davin', 'Jose Miguel Colella']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29108'
versions = ['Python 3.6']