Help: rewrite intro of add_custom_target

The current doc page says: "The target has no output file." This is correct only when understood in a precise technical sense: The target has no formal output, but the commands invoked by the target are well allowed to generate output files.

Furthermore, the doc page says: "it will always be built." This is only true if the flag ALL is given. Otherwise, the target will only be built when explicitly invoked.

This is my attempt to describe the purpose of the command in a simple, yet precise way. Feel free to edit the wording.