Clarify documentation for binary arithmetic operation subclass __r*__ precedence

[brettcannon]

BPO	41584
Nosy	@brettcannon, @vedgar, @miss-islington
PRs	bpo-41584: clarify when the reflected method of a binary arithemtic operator is called #22505 [3.9] bpo-41584: clarify when the reflected method of a binary arithemtic operator is called (GH-22505) #22568 [3.8] bpo-41584: clarify when the reflected method of a binary arithemtic operator is called (GH-22505) #22569

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/brettcannon'
closed_at = <Date 2020-10-05.18:34:03.697>
created_at = <Date 2020-08-18.17:56:34.350>
labels = ['3.8', '3.9', '3.10', 'docs']
title = 'Clarify documentation for binary arithmetic operation subclass __r*__ precedence'
updated_at = <Date 2020-10-05.18:34:03.697>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2020-10-05.18:34:03.697>
actor = 'brett.cannon'
assignee = 'brett.cannon'
closed = True
closed_date = <Date 2020-10-05.18:34:03.697>
closer = 'brett.cannon'
components = ['Documentation']
creation = <Date 2020-08-18.17:56:34.350>
creator = 'brett.cannon'
dependencies = []
files = []
hgrepos = []
issue_num = 41584
keywords = ['patch']
message_count = 8.0
messages = ['375621', '375622', '375645', '375646', '375668', '378063', '378065', '378067']
nosy_count = 5.0
nosy_names = ['brett.cannon', 'docs@python', 'veky', 'miss-islington', 'Eric Wieser']
pr_nums = ['22505', '22568', '22569']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue41584'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[brettcannon]

https://docs.python.org/3/reference/datamodel.html#object.\_\_ror__ has a note saying:

"If the right operand's type is a subclass of the left operand's type and that subclass provides the reflected method for the operation, this method will be called before the left operand's non-reflected method." The slightly unclear part (at least to me) is the "provides the reflected method."

What this actually means according to https://bugs.python.org/issue30140 is that the subclass implements the __r*__ method, not just that the method is reachable on the subclass via getattr(). That wasn't clear to me when I initially read this.