6914986: Make sure openjdk doc generation not turned off with JDK_UPDATE_VERSION

Summary: Only turn off documentation for updates when not building OpenJDK
Reviewed-by: ohair

make/Defs-internal.gmk

@@ -231,10 +231,12 @@ endif
 ifndef NO_DOCS
   # Default value (we want javadoc run)
   GENERATE_DOCS=true
-  # No DOCS build when JDK_UPDATE_VERSION set
+  # No DOCS build when JDK_UPDATE_VERSION set on non-OPENJDK builds
+  ifndef OPENJDK
     ifdef JDK_UPDATE_VERSION
       GENERATE_DOCS=false
     endif
+  endif
   # If langtools, corba, jaxp, and jaxws are not being built, 
   #   a full jdk javadoc is not possible
   ifneq ($(BUILD_LANGTOOLS), true)