Conflict resolution documentation

[philipbeber]

Describe the bug
The paragraph in the introduction describing conflict resolution stops mid-sentence: By default, only one lock for a given hash can be acquired. What happens when a lock can't be acquired is governed by a chosen [Conflict Strategy](https://github.com/mhenrixon/sidekiq-unique-jobs#conflict-strategy) strategy. Unless a conflict strategy is chosen

Expected behavior
It would be nice to know:

• What happens when a conflict strategy is not chosen, i.e. what is the default behavior?
• How does one simply ignore queuing a job if the lock is already taken (rather than sending it to the morgue)?
• This text: By default, only one lock for a given hash can be acquired implies there's a way to have more than one lock for each hash. Is this a reference to the lock_args feature or something else?

Additional context
Great gem! Thanks for providing this. It is very useful.

[exterm]

This would indeed be very helpful. I've spent some time reading through the code and it looks like the default confliect resolution strategy is [the null strategy(https://github.com/mhenrixon/sidekiq-unique-jobs/blob/main/lib/sidekiq_unique_jobs/on_conflict/null_strategy.rb). However, I'm not quite sure what behavior that entails.