depends: Add comment about cache invalidation

depends/Makefile

@@ -127,6 +127,8 @@ $(host_arch)_$(host_os)_id_string+=$(shell $(host_RANLIB) --version 2>/dev/null)
 $(host_arch)_$(host_os)_id_string+=$(shell $(host_STRIP) --version 2>/dev/null)
 
 ifneq ($(strip $(FORCE_USE_SYSTEM_CLANG)),)
+# Make sure that cache is invalidated when switching between system and
+# depends-managed, pinned clang
 build_id_string+=system_clang
 $(host_arch)_$(host_os)_id_string+=system_clang
 endif