Makefile: explain why we do not use the default

Hopefully it will prevent future mistakes.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

Makefile

@@ -807,6 +807,8 @@ KBUILD_CFLAGS += -Os
 KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := z
 endif
 
+# Always set `debug-assertions` and `overflow-checks` because their default
+# depends on `opt-level` and `debug-assertions`, respectively.
 KBUILD_RUSTFLAGS += -Cdebug-assertions=$(if $(CONFIG_RUST_DEBUG_ASSERTIONS),y,n)
 KBUILD_RUSTFLAGS += -Coverflow-checks=$(if $(CONFIG_RUST_OVERFLOW_CHECKS),y,n)
 KBUILD_RUSTFLAGS += -Copt-level=$\