Explain why JIRA DVCS requires API write-access

What does this MR do?

The DVCS connector uses the application token over GitLab API calls to perform some of its required webhook configuration.

For this to succeed, the wider api scope needs to be selected instead of the limited read_api scope.

The integration cannot function successfully if the read-only scope is provided instead.

This change adds a nested line in the instructions that setup the scope explaining why the scope is required by the connector.

The doc-note was added after discussion over Slack with the connector devs over channel #f_jira: https://gitlab.slack.com/archives/C2G2VTZPA/p1635943124008300?thread_ts=1634890224.004900&cid=C2G2VTZPA

Related issues

Issue: #343942 (closed)

Customer tickets:

• https://gitlab.zendesk.com/agent/tickets/245066
• https://gitlab.zendesk.com/agent/tickets/213984

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