qemu-options.hx: 9p: clarify -virtfs vs. -fsdev

The docs are ambiguous about the difference (or actually their
equality) between options '-virtfs' vs. '-fsdev'. So clarify that
'-virtfs' is actually just a convenience shortcut for its
generalized form '-fsdev' in conjunction with '-device virtio-9p-pci'.

And as we're at it, also be a bit more descriptive what 9pfs is
actually used for.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <208f1fceffce2feaf7c900b29e326b967dce7762.1585661532.git.qemu_oss@crudebyte.com>
Signed-off-by: Greg Kurz <groug@kaod.org>

qemu-options.hx

@@ -1542,9 +1542,17 @@ SRST
 ``-virtfs proxy,sock_fd=sock_fd,mount_tag=mount_tag [,writeout=writeout][,readonly]``
   \
 ``-virtfs synth,mount_tag=mount_tag``
-    Define a new filesystem device and expose it to the guest using a
-    virtio-9p-device. The general form of a Virtual File system
-    pass-through options are:
+    Define a new virtual filesystem device and expose it to the guest using
+    a virtio-9p-device (a.k.a. 9pfs), which essentially means that a certain
+    directory on host is made directly accessible by guest as a pass-through
+    file system by using the 9P network protocol for communication between
+    host and guests, if desired even accessible, shared by several guests
+    simultaniously.
+
+    Note that ``-virtfs`` is actually just a convenience shortcut for its
+    generalized form ``-fsdev -device virtio-9p-pci``.
+
+    The general form of pass-through file system options are:
 
     ``local``
         Accesses to the filesystem are done by QEMU.