Add description about linking a vulnerability to existing Jira issues

What does this MR do?

Reason for update:
During my daily work I have activated Jira issue integration feature with selected Enable Jira issue creation from vulnerabilities option. After that some of my project's vulnerabilities were linked to Jira issues and some were not. At the moment there is no information about the reason of this behavior and I had to look inside source code to find the reason. From my point of view it should be mentioned in the docs.

Also I have removed the following information:
"Creating a Jira issue requires that Jira integration is enabled on the project. Note that when Jira integration is enabled, the GitLab issue feature is not available."
Reason:

• to create Jira issue for a vulnerability, "Enable Jira issue creation from vulnerabilities" option should be enabled too. This description presents inside Create a Jira issue for a vulnerability paragraph
• Gitlab issues are still available when Jira integration feature is enabled

Related issues

without issue

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • [Documentation guidelines](https://docs.gitlab.com/ee/develoAt the moment Gitlab issues feature is still available after activation of Jira issues.html#product-tier-badges) is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.