The error-logging change in 81aeab1687 didn't work well for me
in further experiments. Change `interactions`, etc., to have a
`#:no-errors?` argument, instead. Also, add `eval:no-prompt`,
which provides an alternative to `def+int`.
Improve the documentaiton by correcting mistakes and filling
in some missing exports.
When an expression in `examples` or `interactions` raises an
exception, the error message is rendered as part of the documentation.
Now, however, unless the expression is wrapped with `eval:error`, an
error is also logged.
Logging an error is a compromise between backward compatibility (for
documents that rely on undeclared but expected errors) and making a
document fail completely (which would be nicer when an error is not
expected).
