doc: uv_buf_t members are not readonly

This is probably a reminiscent of the usage in libuv 0.x, where alloc_cb
used to return a uv_buf_t.

Refs: https://github.com/libuv/libuv/issues/1027#issuecomment-244386298
PR-URL: https://github.com/libuv/libuv/pull/1033
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Imran Iqbal <imran@imraniqbal.org>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

docs/src/misc.rst

@@ -17,11 +17,11 @@ Data types
 
     .. c:member:: char* uv_buf_t.base
 
-        Pointer to the base of the buffer. Readonly.
+        Pointer to the base of the buffer.
 
     .. c:member:: size_t uv_buf_t.len
 
-        Total bytes in the buffer. Readonly.
+        Total bytes in the buffer.
 
         .. note::
             On Windows this field is ULONG.