Adds haddock example usage of 'in_' operator

[rzetterberg]

I couldn't find an example for how you use the in_ operator and I didn't understand directly how to use it.

So I thought maybe it would help to add a short example for people with less experience like me.

[meteficha]

Nice, thanks! Just a style note, though:

-- Instead of:
'where_' $ 'in_' (person '^.' PersonId) ('valList' personIds)

-- Write this:
'where_' $ person '^.' PersonId `in_` 'valList' personIds

[rzetterberg]

Thanks for the feedback! I have made the suggested changes and squashed the two commits.

[meteficha]

Thanks!