os: add note for the availability of the debugger_present implementation (#24492)

Author: lcheylus

vlib/os/debugger_default.c.v

@@ -1,6 +1,8 @@
 module os
 
 // debugger_present returns a bool indicating if the process is being debugged
+// Note: implementation available only on Darwin, FreeBSD, Linux, OpenBSD and
+// Windows. Otherwise, returns false.
 @[inline]
 pub fn debugger_present() bool {
 	return false