Expand on objectid docstring (#45987)

Author: LilithHafner

base/reflection.jl

@@ -330,7 +330,9 @@ end
 """
     objectid(x) -> UInt
 
-Get a hash value for `x` based on object identity. `objectid(x)==objectid(y)` if `x === y`.
+Get a hash value for `x` based on object identity.
+
+If `x === y` then `objectid(x) == objectid(y)`, and usually when `x !== y`, `objectid(x) != objectid(y)`.
 
 See also [`hash`](@ref), [`IdDict`](@ref).
 """