doc: note buffer lifetime requirements in uv_write

Fixes: https://github.com/libuv/libuv/issues/1072
PR-URL: https://github.com/libuv/libuv/pull/1074
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

docs/src/stream.rst

@@ -173,6 +173,10 @@ API
         uv_write(&req1, stream, a, 2, cb);
         uv_write(&req2, stream, b, 2, cb);
 
+    .. note::
+        The memory pointed to by the buffers must remain valid until the callback gets called.
+        This also holds for :c:func:`uv_write2`.
+
 .. c:function:: int uv_write2(uv_write_t* req, uv_stream_t* handle, const uv_buf_t bufs[], unsigned int nbufs, uv_stream_t* send_handle, uv_write_cb cb)
 
     Extended write function for sending handles over a pipe. The pipe must be