docs: net: fix inaccuracies in msg_zerocopy.rst

Replace "sendpage" with "sendfile". Remove comment about
ENOBUFS when the sockopt hasn't been set; experimentation
indicates that this is not true.

Signed-off-by: nick black <dankamongmen@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/Y/gg/EhIIjugLdd3@schwarzgerat.orthanc
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: dankamongmen

Documentation/networking/msg_zerocopy.rst

@@ -15,7 +15,7 @@ Opportunity and Caveats
 
 Copying large buffers between user process and kernel can be
 expensive. Linux supports various interfaces that eschew copying,
-such as sendpage and splice. The MSG_ZEROCOPY flag extends the
+such as sendfile and splice. The MSG_ZEROCOPY flag extends the
 underlying copy avoidance mechanism to common socket send calls.
 
 Copy avoidance is not a free lunch. As implemented, with page pinning,
@@ -83,8 +83,8 @@ Pass the new flag.
 	ret = send(fd, buf, sizeof(buf), MSG_ZEROCOPY);
 
 A zerocopy failure will return -1 with errno ENOBUFS. This happens if
-the socket option was not set, the socket exceeds its optmem limit or
-the user exceeds its ulimit on locked pages.
+the socket exceeds its optmem limit or the user exceeds their ulimit on
+locked pages.
 
 
 Mixing copy avoidance and copying