docs: clarify undefined behavior in pointer semantics example (fixes …

lesson2/25_B_return_pointer.c

@@ -1,10 +1,19 @@
 #include <stdio.h>
+// WARNING: The get_pointer() function below contains UNDEFINED BEHAVIOR in C.
+// It returns a pointer to a local variable that is destroyed when the function exits.
+// This example is intentionally shown to demonstrate how Go differs from C:
+// Go handles this safely using automatic memory management,
+// while C results in undefined behavior if the pointer is dereferenced.
+
 
 int* get_pointer(void) {
     int i = 42;
     return &i;
 }
 
+// Note: Running this program may appear to work on some systems due to
+// undefined behavior, but it can crash or produce garbage output.
+
 int main(void) {
     int *p = get_pointer();
     printf("%p\n", p);