HDFS-5926 Documentation should clarify dfs.datanode.du.reserved impact from reserved disk capacity (Gabor Bota)

hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml

@@ -349,6 +349,9 @@
       For example, reserved space for RAM_DISK storage can be configured using property
       'dfs.datanode.du.reserved.ram_disk'. If specific storage type reservation is not configured
       then dfs.datanode.du.reserved will be used.
+      Note: In case of using tune2fs to set reserved-blocks-percentage, or other filesystem tools,
+      then you can possibly run into out of disk errors because hadoop will not check those
+      external tool configurations.
   </description>
 </property>