Document cross-compilation aspects of check_..._source_runs() commands
Extracted from this discussion thread.
The documentation currently doesn't state what projects/users need to do if calling the check_..._source_runs()
commands in a cross-compiling scenario. Only the try_run()
command has such docs, but the module commands don't mention how that impacts them. We should document the variables that projects/users need to define when no CMAKE_CROSSCOMPILING_EMULATOR
is set.