Midpoint toggle apparently has a transient hotkey ('A'), which is great, but shouldn't that be visible in the RMB menu of the tool?
I had to search the Hotkeys editor to find out whether it's possible to toggle this on the fly. It's great that it's possible, but I think that the RMB menu should explicitly show that (in a parentheses or italic font for example) or people less inclined to diligently test and search a tool's capabilities will find this tool insufficiently interactive. Non-transient toggles, like current snapping, is the bane of interactivity.
Docs say: “Press the Midpoint hotkey or enable it in the RMB menu”. The default key is not specified, but considering the ever diminishing attention span of the modern human, it's best to start from the assumption that not everyone will read the docs about such a basic modeling tool. Their loss? Only theirs?
Just a thought, with UX/UI in mind.

It probably should mention the hotkey next to the RMB menu entry, but to be clear the hotkey inverts the base operation, which is slightly different than enabling it there via RMB.

Having said that, there is a prompt that says, “Press or Hold the midpoint key (A) to switch between merging at the targets point position or merging at the midpoint.”

Nevertheless, good catch!

ScottKeating
but to be clear the hotkey inverts the base operation, which is slightly different than enabling it there via RMB.

yeah, that's good

ScottKeating
Having said that, there is a prompt that says, “Press or Hold the midpoint key (A) to switch between merging at the targets point position or merging at the midpoint.”

That prompt disappears soon after I activate the tool, it seems, and as such I missed it. I don't know how to reproduce the issue since it's not happening all the time. It may be related to dual display setups, I'll surely file a bug report if I find the cause.