Makefile: add comment to explain RUSTC_BOOTSTRAP

Link: https://lore.kernel.org/lkml/CAK7LNASzund0a=t6gz3Mfex3j6jMfjYW--nqv9x_wY=RzSR-=g@mail.gmail.com/
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Makefile

@@ -593,6 +593,7 @@ ifdef RUST_LIB_SRC
 	export RUST_LIB_SRC
 endif
 
+# Allows the usage of unstable features in stable compilers.
 export RUSTC_BOOTSTRAP := 1
 
 export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE LD CC HOSTPKG_CONFIG