HADOOP-16759. FileSystem Javadocs to list what breaks on API changes

Followup to the main openFile().withStatus() patch.
It turns out that this broke the hive builds, which
was not well appreciated.

This patch lists places to review in the hadoop codebase,
and external projects where changes are likely to cause problems.

Contributed by Steve Loughran

Change-Id: Ifac815c65b74d083cd277764b780ac2b5b0f6b36

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/FileSystem.java

@@ -132,6 +132,25 @@
  * New methods may be marked as Unstable or Evolving for their initial release,
  * as a warning that they are new and may change based on the
  * experience of use in applications.
+ * <b>Important note for developers</b>
+ *
+ * If you're making changes here to the public API or protected methods,
+ * you must review the following subclasses and make sure that
+ * they are filtering/passing through new methods as appropriate.
+ *
+ * {@link FilterFileSystem}: methods are passed through.
+ * {@link ChecksumFileSystem}: checksums are created and
+ * verified.
+ * {@code TestHarFileSystem} will need its {@code MustNotImplement}
+ * interface updated.
+ *
+ * There are some external places your changes will break things.
+ * Do co-ordinate changes here.
+ *
+ * HBase: HBoss
+ * Hive: HiveShim23
+ * {@code shims/0.23/src/main/java/org/apache/hadoop/hive/shims/Hadoop23Shims.java}
+ *
  *****************************************************************/
 @SuppressWarnings("DeprecatedIsStillUsed")
 @InterfaceAudience.Public