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.