Better docs for custom spy strategies

[sgravrock]

Right now all we have is https://jasmine.github.io/api/edge/jasmine.html#.addSpyStrategy which doesn't explain what a strategy is or what the factory should return. It would be good to have something that explains what custom spy strategies are useful for and how to write them.

Some context:
https://groups.google.com/g/jasmine-js/c/kdzzuNxOFEU/m/1pALAoT63EsJ
jasmine/jasmine#279
jasmine/jasmine#94
jasmine/jasmine@4934e42