I thought it would be helpful to be able to place a clickable link to another tag that might be related or confused with the current one...
We can not use markup or links in the tag wiki excerpts shown as a pop-up on hovering over a tag, or when tagging a question. Still, we should - in plain text - make a short reference there for similar tags when they may be confused. Similar tags, and synonymized tags shown up in a list to select from when we tag a question.
This is different in the actual tag wiki where we can use all formatting available just like in questions or answers (including external links, images, etc.)
See for example the following tags, were I put a reference to a similar tag in both, the excerpt, and the wiki. In the wiki I referenced the corresponding tag using