[9.x] Add `withWhereHas` documentation to Eloquent

[ashleyshenton]

I came across this method the other day and have had many use cases for it but was unaware it existed. This PR adds a mention to the docs. I'm not sure where it is best to go, whether with Querying Relationships or Eager Loading as it covers both. I've tried to keep the example relevant with other examples in the eager loading section.

https://github.com/laravel/framework/pull/42597