reference docs don't explain givens with ConstrApp

[bjornregnell]

Compiler version

3.3.1

Minimized code

This way of writing givens is a bit surprising:

given ops1: IntOps()

which perhaps is a shorthand for

given ops1: IntOps with {}

Expectation

An explanation and example of the usage of given IntOps() should be given here:
https://github.com/lampepfl/dotty/blob/main/docs/_docs/reference/contextual/givens.md