target_compile_definition documentation issue

Original discourse post: https://discourse.cmake.org/t/does-target-compile-definition-assign-a-value-to-the-item/987

Essentially I think the cmake documentation should mention there is no guarantee of a value being assigned to a compile definition.

Our project heavily relies on the use of the "#if" construct so it is important to know what cmake will/won't do.

Interestingly it hasn't been strictly necessary to specify since it's been passing our CI without issue so far. So we have been relying on undefined behavior (scary).

Our current testing primarily uses:

• GCC, Unix Makefiles
• MSVC, Ninja