Explain why JIRA DVCS requires API write-access

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.

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/343942

doc/integration/jira/dvcs.md

@@ -86,6 +86,7 @@ you can set up this integration with your own account instead.
      `https://<gitlab.example.com>/-/jira/login/oauth/callback`.
 
 1. For **Scopes**, select `api` and clear any other checkboxes.
+   - The connector requires a _write-enabled_ `api` scope to automatically create and manage required webhooks.
 1. Select **Submit**.
 1. GitLab displays the generated **Application ID**
    and **Secret** values. Copy these values, as you need them to configure Jira.