docs: Better explain the requirements for @PlanningId properties (#1529)

The current text states that an `@PlanningId` property must implement
`Object#equals()` and `Object#hashCode()`. If they are instantiable
types, they automatically implement those methods, although not in
accordance with the respective contracts.

Author: mthmulders

docs/src/modules/ROOT/pages/using-timefold-solver/modeling-planning-problems.adoc

@@ -196,7 +196,7 @@ A `@PlanningId` property must be:
 * Unique for that specific class
 ** It does not need to be unique across different problem fact classes
 (unless in that rare case that those classes are mixed in the same value range or planning entity collection).
-* An instance of a type that implements `Object.hashCode()` and `Object.equals()`.
+* An instance of a type that implements `Object.hashCode()` and `Object.equals()`. See the Javadoc on the https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Object.html[`java.lang.Object` class] for details.
 ** It's recommended to use the type `Integer`, `int`, `Long`, `long`, `String` or `UUID`.
 * Never `null` by the time `Solver.solve()` is called.
 
@@ -3076,4 +3076,4 @@ public interface TestdataBaseEntity {
     ...
 }
 ----
-====
+====