qapi/block-core.json: Remove stale description of 'blockdev-add'

On a 'qemu-discuss' thread[1], Kevin identifies that the current doc
blurb for @blockdev-add is stale:

    This is actually a documentation bug. @id doesn't exist,
    blockdev-add never creates a BlockBackend. This was different in the
    very first versions of the patches to add blockdev-add and we
    probably just forgot to update the documentation after removing it.

So remove the stale bits.

And the requirement for 'node-name' is already mentioned in the
documentation of @BlockdevOptions:

    [...]
    # @node-name: the node name of the new node (Since 2.0).
    #             This option is required on the top level of blockdev-add.
    #             Valid node names start with an alphabetic character and may
    #             contain only alphanumeric characters, '-', '.' and '_'. Their
    #             maximum length is 31 characters.
    [...]

[1] https://lists.nongnu.org/archive/html/qemu-discuss/2020-07/msg00071.html


    -- equivalent to "-drive if=ide,id=disk0....."

Fixes: be4b67bc ("blockdev: Allow creation of BDS trees without BB")

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Suggested-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200805100158.1239390-1-kchamart@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>