doc: Add '@' operator entry to index

[andresdelfino]

BPO	33847
Nosy	@terryjreedy, @andresdelfino, @miss-islington
PRs	bpo-33847: Add '@' operator entry to index #7669 [3.7] bpo-33847: Add '@' operator entry to index (GH-7669) #7717 [3.6] bpo-33847: Add '@' operator entry to index (GH-7669) #7718

^{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/terryjreedy'
closed_at = <Date 2018-06-15.20:12:05.440>
created_at = <Date 2018-06-12.18:44:14.135>
labels = ['3.8', 'type-feature', '3.7', 'docs']
title = "doc: Add '@' operator entry to index"
updated_at = <Date 2018-06-15.20:12:05.439>
user = 'https://github.com/andresdelfino'

bugs.python.org fields:

activity = <Date 2018-06-15.20:12:05.439>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = True
closed_date = <Date 2018-06-15.20:12:05.440>
closer = 'terry.reedy'
components = ['Documentation']
creation = <Date 2018-06-12.18:44:14.135>
creator = 'adelfino'
dependencies = []
files = []
hgrepos = []
issue_num = 33847
keywords = ['patch']
message_count = 5.0
messages = ['319398', '319633', '319651', '319657', '319658']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'docs@python', 'adelfino', 'miss-islington']
pr_nums = ['7669', '7717', '7718']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue33847'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[andresdelfino]

Currently, the index only refers to decorators for the @ symbol. PR adds entry for the matrix multiplication operator.

[terryjreedy]

This is definitely an oversight to be corrected. Index entries for the other binary arithmetic operators point to the table at
https://docs.python.org/3/library/stdtypes.html#numeric-types-int-float-complex
But @ is not in that table since number @ number is meaningless and invalid. So I agree that the best location is the sentence in
https://docs.python.org/3/reference/expressions.html#binary-arithmetic-operations.

[terryjreedy]

New changeset 6951186 by Terry Jan Reedy (Andrés Delfino) in branch 'master':
bpo-33847: Add '@' operator entry to index (GH-7669)
6951186

[miss-islington]

New changeset c05c0e0 by Miss Islington (bot) in branch '3.7':
bpo-33847: Add '@' operator entry to index (GH-7669)
c05c0e0

[miss-islington]

New changeset cd9ede6 by Miss Islington (bot) in branch '3.6':
bpo-33847: Add '@' operator entry to index (GH-7669)
cd9ede6