router.field support for compositeId router

Although there is code to support router.field for CompositeId, it only calculates a simple (non-compound) hash, which isn't that useful unless you don't use compound ids (this is why I changed the docs to say router.field is only supported for the implicit router). The field value should either

• be used to calculate the full compound hash
• be used to calculate the prefix bits, and the uniqueKey will still be used for the lower bits.

For consistency, I'd suggest the former.
If we want to be able to specify a separate field that is only used for the prefix bits, then perhaps that should be "router.prefixField"