Clarify gpg.ssh.allowedSignersFile for ssh key signing

[nguyenvulong]

Code of Conduct

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What article on docs.github.com is affected?

https://docs.github.com/en/authentication/managing-commit-signature-verification/telling-git-about-your-signing-key?platform=linux#telling-git-about-your-ssh-key

What part(s) of the article would you like to see updated?

Telling Git about your SSH key

I believe it should mention adding

gpg.ssh.allowedsignersfile=/path/to/.config/git/allowed_signers

Otherwise, git would complain that

error: gpg.ssh.allowedSignersFile needs to be configured and exist for ssh signature verification

and in case rules were set, github will refuse push

remote: - Commits must have verified signatures.
remote:   Found 1 violation:
remote:
remote:   bd96ff44bfa007357c164fb564b3fdd781b31322
remote:
To github.com:just/a-repo.git
 ! [remote rejected] main -> main (push declined due to repository rule violations)
error: failed to push some refs to 'github.com:just/a-repo.git'

Additional information

Related issue #28577

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[nguyenalex836]

@nguyenvulong Thank you for opening an issue! I'll get this triaged for review ✨