I hit a problem with `epkgs.vterm` from `nixos-20.09` being too old and thus missing `vterm-goto-char` which is present in newer versions of `epkgs.vterm`.
I added a note to clear this up for future users, and added two other changes/clarifications as well:
* Changed `epkgs.emacs-libvterm` -> `epkgs.vterm`, as the old name is deprecated.
* Added note that `epkgs.vterm` already contains a `vterm-module.so`
* Added note that the version of `emacs` needs to be recent enough and that otherwise bugs might occur.
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