iov: Improve documentation of iov_skip_bytes()

As pointed out in review, the documentation comments for iov_skip_bytes()
are more confusing than they should be.  Reword them, including updating
parameter names, to make it clearer.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

iov.h

@@ -19,7 +19,7 @@
 #include <string.h>
 
 size_t iov_skip_bytes(const struct iovec *iov, size_t n,
-		      size_t vec_offset, size_t *buf_offset);
+		      size_t skip, size_t *offset);
 size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
                     size_t offset, const void *buf, size_t bytes);
 size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,