YARN-11657. Remove protobuf-2.5 as dependency of hadoop-yarn-api

Updated building docs.

Change-Id: I50b755d129fa0a7841edb787f430a48ddb3e8cf1

BUILDING.txt

@@ -315,12 +315,12 @@ Controlling the redistribution of the protobuf-2.5 dependency
 
     The protobuf 2.5.0 library is used at compile time to compile the class
     org.apache.hadoop.ipc.ProtobufHelper; this class known to have been used by
-    external projects in the past. Protobuf 2.5 is not used elsewhere in
+    external projects in the past. Protobuf 2.5 is not used directly in
     the Hadoop codebase; alongside the move to Protobuf 3.x a private successor
     class, org.apache.hadoop.ipc.internal.ShadedProtobufHelper is now used.
 
     The hadoop-common module no longer exports its compile-time dependency on
-    protobuf-2.5. Hadoop distributions no longer include it.
+    protobuf-java-2.5.
     Any application declaring a dependency on hadoop-commmon will no longer get
     the artifact added to their classpath.
     If is still required, then they must explicitly declare it:
@@ -337,10 +337,14 @@ Controlling the redistribution of the protobuf-2.5 dependency
 
            -Dcommon.protobuf2.scope=compile
 
-    If this is done then protobuf-2.5.0.jar will again be exported as a
+    If this is done then protobuf-java-2.5.0.jar will again be exported as a
     hadoop-common dependency, and included in the share/hadoop/common/lib/
     directory of any Hadoop distribution built.
 
+    Note that protobuf-java-2.5.0.jar is still placed in
+    share/hadoop/yarn/timelineservice/lib; this is needed by the hbase client
+    library.
+
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 Building components separately