DOC: config: mention the limitation on server id range for consistent hash

DOC: config: mention the limitation on server id range for consistent hash

When using "hash-type consistent", we default to using the server's ID
as the insertion key. However, that key is scaled to avoid collisions
when inserting multiple slots for a server (16 per weight unit), and
that scaling loses the 4 topmost bits of the ID, so the only effective
range of IDs is 1..268435456, and anything above will provide the same
hashing keys again.

Let's mention this in the documentation, and also remind that it can
affect "balance random". This can be backported to all versions.