User's guide: Clarify overlapping instance candidate elimination

This MR updates the user's guide section on overlapping instance candidate elimination to use "or" verbiage instead of "either/or" in regards to the current pair of candidates' being overlappable or overlapping. "Either IX is overlappable, or IY is overlapping" can cause confusion as it suggests "Either IX is overlappable, or IY is overlapping, but not both".

This was initially discussed on this Discourse topic:

https://discourse.haskell.org/t/clarification-on-overlapping-instance-candidate-elimination/5677