Query documentation could use an example on how to compare column with dynamic value

Hey!

Stumbled upon another slightly unclear thing — how to filter against a value from a variable. Turns out all of the `filter_` examples in http://tathougies.github.io/beam/user-guide/queries/basic/ use hard-coded values that I guess work due to overloaded strings. When one plugs in a variable, for example, of type `String` to either side, they get a type mismatch. Because `==.` is fairly generic, the error is somewhat backwards — not pushing the `String` towards `QExpr`, but vice versa. As I remembered seeing `Beam.val_` when playing with `insertExpressions`, I thought to try that.

I think it'd be valuable to newcomers if one such example were documented, too. The overloaded string or `Num` variants do for nice examples, but don't address the complexity that later surfaces again.