docs: Don't mention "-mem-path" in multi-process.rst

"-mem-path" corresponds to "memory-backend-file,share=off" and,
therefore, creates a private COW mapping of the file. For multi-proces
QEMU, we need proper shared file-backed memory.

Let's make that clearer.

Message-ID: <20230906120503.359863-9-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

docs/devel/multi-process.rst

@@ -409,8 +409,9 @@ the initial messages sent to the emulation process is a guest memory
 table. Each entry in this table consists of a file descriptor and size
 that the emulation process can ``mmap()`` to directly access guest
 memory, similar to ``vhost_user_set_mem_table()``. Note guest memory
-must be backed by file descriptors, such as when QEMU is given the
-*-mem-path* command line option.
+must be backed by shared file-backed memory, for example, using
+*-object memory-backend-file,share=on* and setting that memory backend
+as RAM for the machine.
 
 IOMMU operations
 ^^^^^^^^^^^^^^^^