hotplug/FreeBSD: document disk hotplug interface

Add the FreeBSD disk hotplug interface details to the block-scripts.txt
document.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>

docs/misc/block-scripts.txt

@@ -42,6 +42,19 @@ argument of the script (i.e. $1).
 The environment variable XENBUS_PATH will be set to the
 path for the block device to be created.
 
+When the script is run, the following nodes shall already have been
+written into xenstore:
+
+ $XENBUS/params    The contents of the 'target' section of the disk specification verbatim.
+ $XENBUS/mode      'r' (for readonly) or 'w' (for read-write)
+
+Inputs (FreeBSD)
+--------------
+
+The scripts are always called with the same set of arguments. The first
+parameter is the xenstore backend path of the device, while the second
+argument is the action, which is always either "add" or "remove".
+
 When the script is run, the following nodes shall already have been
 written into xenstore: