Add inline comment explaining why #undef _res is necessary

Co-authored-by: yhirose <357397+yhirose@users.noreply.github.com>

Author: Copilot

httplib.h

@@ -257,7 +257,7 @@ using socklen_t = int;
 #include <netinet/in.h>
 #ifdef __linux__
 #include <resolv.h>
-#undef _res
+#undef _res // Undefine _res macro to avoid conflicts with user code (#2278)
 #endif
 #include <csignal>
 #include <netinet/tcp.h>