A recent tag synonym suggestion issue we were having on Movies.SE resulted in finding out that the help Center page for the tag synonym privilege is not clear.
I previously thought that the five person vote (1 to suggest, 4 minimum upvotes) for a tag synonym would automatically and silently retag old questions already bearing the synonym. I was told this was not the case. That they will need manual Merging by the mods.
Reading the help page, in hindsight, seems to say that only when a new question is tagged with the synonym that it will be changed. It does not explicitly mention the old tags stay the same.
My suggestion is to update the help center/privilege page and the tag synonym suggestion page to explicitly state that old questions won't get retagged, that a meta question requesting merging should be asked instead. Just to clarify.
I'm sure this results in a few pointless tag synonym requests, as some people think it counts for all questions, not just new ones.