[Feature/Enhancement] Add hint to MacOS external editor command

[shlemiel]

I use macOS, not the most recent version though: Monterey (12.6.3).

In my macOS, trying to execute external editor just by its commands does not work. Particularly, I use 'VimR' and inputting 'vimr' in 'External editor command' does not work. When I press Command-X, something is displayed in status bar and nothing executes, and then the focus go back to the mindforger right away.

To resolve this, I use the command 'open -W -n -a vimr --args' like shown in the screenshot. I recommend this to be written in somewhere, user documentation for example.

There is a slight bug in this situation though (macOS or VimR itself seem to be responsible of it): after the VimR process is run, just typing ":wq" does not terminate the VimR app - when in the case, VimR keeps remain in the app bar - the user explicitly have to type Command-Q to terminate the program.