Clarify how to handle two pinned-by-default items

What does this MR do?

Based on this conversation between @msedlakjakubowski and me, let's make clear that the two pinned-by-default items (Issues and Merge requests should have their left nav access documented the long way around, even though there's a shorter way to do it.

tl;dr: always use **Code > Merge requests** even though **Merge requests** is available by default on the sidebar, because that option can be changed by users

Related issues

Related to Begin updating the content of the create-MR page (!139287 - merged) where this came up

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