Ways of estimating progress through checking are not clearly described for all checking roles

[thejayps]

See https://github.com/Ravenbrook/mps/pull/214#issuecomment-1593155571

Checking guidance for each checking role should probably describe:

• how to estimate how much work there is in total for the checker, and
• how to estimate progress.

The expected range of accuracies of the estimates possible for each checking role should be discussed in the guidance so checkers know what is an acceptable estimate. In cases where it's impossible to give guidance that is mostly universally applicable to the role, we should say that this is the case and invite the checker to use an estimation method that seems appropriate for the situation and report on the accuracy they expect their estimate has.