[3.6] bpo-31567: add or fix decorator markup in docs (GH-3959) (GH-3966)

[miss-islington]

(cherry picked from commit 0e61e67)

https://bugs.python.org/issue31567

[the-knights-who-say-ni]

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[miss-islington]

@dmiyakawa and @merwok: Backport status check is done, and it's a success ✅ .

[miss-islington]

@dmiyakawa and @merwok: Backport status check is done, and it's a success ✅ .

[merwok]

@berkerpeksag Here it is!

[berkerpeksag]

LGTM, thanks!

[miss-islington]

Thanks, @merwok!