User's guide: Clarify overlapping instance candidate elimination

This commit 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

docs/users_guide/exts/instances.rst

@@ -486,8 +486,8 @@ like this:
    -  :math:`IY` is strictly more specific than :math:`IX`.  That
       is, :math:`IY` is a substitution instance of :math:`IX` but not vice versa.
 
-   -  Either :math:`IX` is *overlappable*, or :math:`IY` is *overlapping*. (This
-      "either/or" design, rather than a "both/and" design, allow a
+   -  :math:`IX` is *overlappable* or :math:`IY` is *overlapping*. (This
+      "or" design, rather than an "and" design, allows a
       client to deliberately override an instance from a library,
       without requiring a change to the library.)