Try to improve the documentation on registered buffers

It seems to have caused confusion regarding the relation between io_uring
and the fellow memory cache, which exists only to the extent that io_uring
manages IO in and out of it.

src/vmod_slash.vcc

@@ -791,7 +791,9 @@ FELLOW KNOWN ISSUES
 * On Linux with ``io_uring``, by default, `fellow` registers all of
   the memory cache as buffers using
   :ref:`io_uring_register_buffers(3)` to achieve optimal performance
-  at runtime, if supported by the system.
+  at runtime, if supported by the system. Where supported, this
+  enables *zero-copy* IO, where the hardware performs DMA directly
+  into the `fellow` memory cache.
 
   Buffer registrations happen in multiple threads in parallel, one for
   each io ring.
@@ -811,6 +813,9 @@ FELLOW KNOWN ISSUES
   registration is forced to be done as serial, syncronous registration
   operations.
 
+  Note that even with registered buffers, ``io_uring`` has nothing to
+  do with how the `fellow` memory cache and LRU on it work.
+
 * If initialization takes longer than the :ref:`varnishd(1)`
   ``cli_timeout``, it might not come up properly in that it does not
   accept connections after finishing storage initialization. To avoid