Update Jira DVCS documentation to take real time sync into account

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GitLab or GitLab Self-Managed DVCS supports real time sync based on webhooks. The documentation still says:

Configure this connection when you want to import all GitLab commits and branches, for the groups you specify, into Jira. This import takes a few minutes and, after it completes, refreshes every 60 minutes

• https://docs.gitlab.com/ee/integration/jira/dvcs.html#configure-jira-for-dvcs
• https://docs.gitlab.com/ee/integration/jira/dvcs.html#refresh-data-imported-to-jira

I've created this issue based on the discussion here: #3359 (comment 911490930)