make(1): document the value restrictions for Boolean variables

The previous lenient rule came from the sprite.h header that was not
specific to make.  To avoid confusion, only the expected values should
be stored in a Boolean variable.  To help find obvious violations and
inconsistencies, there are different possibilities for the Boolean type,
during development.

In C there is no way to actually enforce this restriction at runtime.
It would be possible in C++, but the code is not ready to be compiled
with a C++ compiler.

(rillig)