Documentation of the `reif` module

[constraintAutomaton]

Currently, the reif module mostly just links to a paper (which, to be fair, is quite approachable and helpful for understanding how the predicates work). However, I think it would be much nicer if the documentation were included inline with the doc because it is more convenient and feel less intimidating.

[triska]

For everyone interested in working on this, please see the existing discussion for example in: https://github.com/mthom/scryer-prolog/pull/2407

[UWN]

Did you understand 7 General reification?