ExternalProject: Clarify `BYPRODUCTS` docs

Clarify that the `BUILD_BYPRODUCTS` and `BYPRODUCTS` options
in ExternalProject may also be required when using the Ninja
generator (as that's one of the primary reasons why you'd want to
use those options).

I've also mentioned that `add_custom_command` has additional
documentation on what `BYPRODUCTS` does.