default.yaml: stop discouraging writable mounts (for 9p and virtiofs)

Setting `writable: true` is still discouraged for reverse-sshfs due to
potential connectivity issues, but it should be fine for 9p (virtio-9p-pci)
and virtiofs.

Signed-off-by: Akihiro Suda <[email protected]>

Author: AkihiroSuda

examples/default.yaml

@@ -71,8 +71,7 @@ mounts:
   # Configure the mountPoint inside the guest.
   # 🟢 Builtin default: value of location
   mountPoint: null
-  # CAUTION: `writable` SHOULD be false for the home directory.
-  # Setting `writable` to true is possible, but untested and dangerous.
+  # Setting `writable` to true is discouraged when mountType is set to "reverse-sshfs".
   # 🟢 Builtin default: false
   writable: null
   sshfs: