You will need to go into your field configurations and change the renderer (from default style) to wiki style renderer for fields such as comments, descriptions, environment, summary etc.
here is the link
What markup? Are you talking about the JIRA Wiki Renderer? Do you want a custom field to have this ability? if so, you can just follow the steps here (https://confluence.atlassian.com/jira/configuring-renderers-185729478.html)
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