The following issues are reported regarding headings in documentation comments.
For a general background information, see this message to jdk-dev@ojn.
   https://mail.openjdk.java.net/pipermail/jdk-dev/2019-March/002671.html

In the following messages ...

"missing heading" means that a heading is more than one greater than the
previous heading or the generated heading that precedes this text in the
generated page.. To fix this, lower the rank in the heading tag.
(e.g. change <h3> to <h2>).

"low" heading means that a heading is less than or equal to the generated
heading that precedes this text on the generated page. To fix this, increase
the rank in the heading tag. (e.g. change <h1> to <h2> or <h4> depeding on
the context.)

open/src/java.base/share/classes/java/util/concurrent/ExecutorService.java:69: Note: missing heading <h2>
 * <h3>Usage Examples</h3>
   ^
open/src/java.base/share/classes/java/util/concurrent/locks/Condition.java:141: Note: missing heading <h2>
 * <h3>Implementation Considerations</h3>
   ^
open/src/java.base/share/classes/java/util/concurrent/locks/Lock.java:120: Note: missing heading <h2>
 * <h3>Memory Synchronization</h3>
   ^
open/src/java.base/share/classes/java/util/concurrent/locks/AbstractQueuedSynchronizer.java:107: Note: missing heading <h2>
 * <h3>Usage</h3>
   ^
open/src/java.base/share/classes/java/util/concurrent/locks/ReentrantReadWriteLock.java:207: Note: missing heading <h2>
 * <h3>Implementation Notes</h3>
   ^
open/src/java.base/share/classes/java/util/concurrent/ScheduledExecutorService.java:68: Note: missing heading <h2>
 * <h3>Usage Example</h3>