documentation of add_custom_target: describe possible values of COMMAND argument
The CMake commands add_custom_target and add_custom_command (and others?) have an argument COMMAND. The documentation currently does not say what sort of commands are meant to be valid arguments here. After long research, I found that a partial answer is provided by "cmake -E help". It would certainly be helpful if this and more information were provided, or linked to, on the doc pages of add_custom_target, add_custom_command etc.