RFR: 8258776: ThreadLocal#initialValue() Javadoc is unaware of ThreadLocal#withInitial()

[Jaikiran Pai]

Can I please get a review of this doc only change which addresses the javadoc issue noted in https://bugs.openjdk.org/browse/JDK-8258776? 

As noted in that issue, the `ThreadLocal.initialValue()` API javadoc suggests subclassing `ThreadLocal` and overriding the `initialValue()` method instead of recommending the `withInitial` method that was added in Java 8.

The commit here updates the javadoc of `initialValue()` method to note that `withInitial` method is available for use if the programmer wants to provide an initial value. The updated javadoc continues to note that the `ThreadLocal` can be subclassed and the method overriden as the other way of providing an initial value. This change now uses the `@implSpec` construct to state this default behaviour of the `initialValue()` method.

Additionally, the `get()` method's javadoc has also been updated to account for the semantics of `initialValue()`. In fact, in its current form, before the changes in this PR, the `get()` method states:

> If the current thread does not support thread locals then
> this method returns its {@link #initialValue} (or {@code null}
> if the {@code initialValue} method is not overridden).

which isn't accurate, since the `get()` will return a non-null value if the ThreadLocal was constructed through `ThreadLocal.withInitial`. Here's a trivial code which contradicts this current javadoc:


public class Test {
  public static void main(final String[] args) throws Exception {
    Thread.ofPlatform().name("hello").allowSetThreadLocals(false).start(() -> {
	var t = ThreadLocal.withInitial(() -> 2);
	System.out.println("Thread local value is " + t.get());
    });
  }
}

Running this with `java --enable-preview --source 21 Test.java` returns:

Thread local value is 2


The updated javadoc of `get()` in this PR now matches the behaviour of this method. I believe this will need a CSR, which I'll open once we have an agreement on these changes.

Furthermore, the class level javadoc of `ThreadLocal` has an example of providing an initial value which uses the subclassing strategy. I haven't updated that part. Should we update that to show the `withInitialValue` usage instead (and maybe convert it to a snippet)?

-------------

Commit messages:
 - 8258776: ThreadLocal#initialValue() Javadoc is unaware of ThreadLocal#withInitial()

Changes: https://git.openjdk.org/jdk/pull/11846/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11846&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8258776
  Stats: 9 lines in 1 file changed: 2 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/11846.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11846/head:pull/11846

PR: https://git.openjdk.org/jdk/pull/11846