lease_cmds incorrect error when adding PD lease, lease_cmds doc update

While figuring out and answer for @peterd's question related to SF1892, I found a number of annoying issues:

1. Incorrect error: I was trying to add a PD lease using this command:

{
    "command": "lease6-add",
    "arguments": {
        "subnet-id": 1,
        "ip-address": "2001:db8:1::5",
        "prefix-len": 48,
        "type": "pd",
        "duid": "1a:1b:1c:1d:1e:1f:20:21:22:23:24",
        "iaid": 5678
    }
}

And Kea told me Incorrect lease type: pd, the only supported values are: na, ta and pd. What it should have said "only supported values are IA_NA, IA_PD.

2. The example for lease6-add in Section 16.15.3 uses lease4-add command.

3. There is no example how to delete a lease.

The following basic example can be used:

{
    "command": "lease6-del",
    "arguments": {
        "ip-address": "2001:db8:1::",
        "type": "IA_PD",
        "subnet-id": 1

    }
}

4. If "prefix-len" is specified in the lease6-del, it is ignored. I've added 2001:db8::/48 and then tried to delete 2001:db8::/123 and the deletion command succeeded. I think it should fail.

Adding PD lease:

{
    "command": "lease6-add",
    "arguments": {
        "subnet-id": 1,
        "ip-address": "2001:db8::",
        "prefix-len": 64,
        "type": "IA_PD",
        "duid": "1a:1b:1c:1d:1e:1f:20:21:22:23:24",
        "iaid": 5678
    }
}

deleting PD lease of different length:

{
    "command": "lease6-del",
    "arguments": {
        "ip-address": "2001:db8::",
        "prefix-len": 1234,
        "type": "IA_PD",
        "subnet-id": 1

    }
}

Looks like the whole type parameter is ignored completely. IMHO if not specified, any matching PD lease should be deleted. If specified, the lease should only be deleted if prefix-length matches.