Document that the keys returned by getBoundaryKeys API are not necessarily real keys

The keys chosen for shard boundaries may not be actual keys in the database. For efficiency reasons, it is often the case that these boundaries will be truncated versions of real keys (e.g. the boundary between consecutive keys `abc` and `def` could be `d`).

This has implications for users when keys they store are encoded in some way (e.g. the tuple layer), as they will not be able to reliably decode the boundary keys.