git: openjdk/loom: fibers: Update comment to clarify the most significant tid bits are exclusively reserved for the JVM

[duke]

Changeset: 07848354
Author:    mgronlun <markus.gronlund at oracle.com>
Date:      2021-12-16 13:06:19 +0000
URL:       https://git.openjdk.java.net/loom/commit/07848354fefbfa17fe3741371eba4b0f0caea37c

Update comment to clarify the most significant tid bits are exclusively reserved for the JVM

! src/java.base/share/classes/java/lang/Thread.java