Improved Jira Issues config docs for authentication

[holger]

If you provide the email and Atlassian API token as written, the JiraClient assumes the token is a password and tries to call the API with username (=email) and password (=token). This doesn't work.

Description

Adjusted the config example and added a sentence explaining the two options: a) using a personal access token or b) using email and password to authenticate with the JiraClient.

Checklist

• [ ] The base branch of this PR is preview, rather than main

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[sestinj]

Thanks for the update!