runtime: add note for the availability of the free_memory/0 implementation (#23620)

vlib/runtime/runtime_nix.c.v

@@ -15,6 +15,8 @@ pub fn total_memory() usize {
 }
 
 // free_memory returns free physical memory found on the system.
+// Note: implementation available only on Darwin, FreeBSD, Linux, OpenBSD and
+// Windows. Otherwise, returns 1.
 pub fn free_memory() usize {
 	return free_memory_impl()
 }