Add clarifying help text on legacy issue linked item input about only being able to link Issues (and update docs)

On legacy issuables, only issues can be added in the "Linked items" widget (even though work items can be linked to them by adding the issue to the work item on the work item's detail page).

This limitation is resolved in work items, but in the interim we should clarify the situation for users who are using the legacy issuable, and should ensure this is properly documented.