Explicitly define "inconsistent data space"

[bdice]

Issue Description

The docs don't do a good job of explaining what an "inconsistent" data space means. Users are warned to avoid inconsistency, but it's not clear what that means.

We also need to replace "incoherent" with "inconsistent."

Proposed change (optional)

Describe that consistency essentially means hash(statepoint) == id.

Additional context (optional)

This led to confusion with @cbkerr.