net: add kerneldoc to skb_copy_bits()

Since skb_copy_bits() is called from assembly, add a fat comment to make
clear we should think twice before changing its prototype.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -1369,8 +1369,21 @@ unsigned char *__pskb_pull_tail(struct sk_buff *skb, int delta)
 }
 EXPORT_SYMBOL(__pskb_pull_tail);
 
-/* Copy some data bits from skb to kernel buffer. */
-
+/**
+ *	skb_copy_bits - copy bits from skb to kernel buffer
+ *	@skb: source skb
+ *	@offset: offset in source
+ *	@to: destination buffer
+ *	@len: number of bytes to copy
+ *
+ *	Copy the specified number of bytes from the source skb to the
+ *	destination buffer.
+ *
+ *	CAUTION ! :
+ *		If its prototype is ever changed,
+ *		check arch/{*}/net/{*}.S files,
+ *		since it is called from BPF assembly code.
+ */
 int skb_copy_bits(const struct sk_buff *skb, int offset, void *to, int len)
 {
 	int start = skb_headlen(skb);