== docs: strengthen link between hashing and equality

ref #55134.

base/operators.jl

@@ -111,9 +111,10 @@ Use [`isequal`](@ref) or [`===`](@ref) to always get a `Bool` result.
 New numeric types should implement this function for two arguments of the new type, and
 handle comparison to other types via promotion rules where possible.
 
-[`isequal`](@ref) falls back to `==`, so new methods of `==` will be used by the
-[`Dict`](@ref) type to compare keys. If your type will be used as a dictionary key, it
-should therefore also implement [`hash`](@ref).
+Equality and hashing are intimately related; two values that are considered [`isequal`](@ref) **must**
+have the same [`hash`](@ref). If a type customizes the behavior of `==` and/or [`isequal`](@ref),
+then [`hash`](@ref) must be similarly implemented. `Set`s, `Dict`s, and many other internal
+implementations assume that this invariant holds.
 
 If some type defines `==`, [`isequal`](@ref), and [`isless`](@ref) then it should
 also implement [`<`](@ref) to ensure consistency of comparisons.