doc/gatt-api: Add Handle property

doc/gatt-api: Add Handle property

When acting as server it is useful to select where to allocate the
handle for an attribute so it can be restored in the same position when
restarting the daemon or rebooting the system.

In order to do that the application also needs to know in which handle
the attribute is allocated the very first time it is registered, this
also allows for a better integration with PTS and tools like auto-pts
which needs to know the handles where the attributes have been
allocated.

Change-Id: I298c07b8e6e6cb1bc163412b5c7acaed9a64b004
Signed-off-by: himanshu <h.himanshu@samsung.com>