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Microsoft wrote a protocol to unify these kind of technologies, making it easier to implement these functions and support for more programming-languages in any text editor or IDE. This (open source) protocol is called Language Server Protocol - wiki. It allows for auto-completion of code like you described and way, way more. It is implemented in programs like VS Code and can also be implemented in Vim, Neovim and a lot more. I use it in Neovim myself (which has a lot of plugins available based on the protocol).
Suprised that no one has mentioned this, but upterm seems to be exactly what you're describing--a terminal emulator that shows a drop-down list of suggestions with explanations. Sadly, only a few commands are supported, and it's no longer being worked on.