HDFS-12770. Add doc about how to disable client socket cache. Contributed by Weiwei Yang.

Signed-off-by: Wei-Chiu Chuang <weichiu@apache.org>

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

@@ -3907,6 +3907,7 @@
   <value>16</value>
   <description>
     Socket cache capacity (in entries) for short-circuit reads.
+    If this value is set to 0, the client socket cache is disabled.
   </description>
 </property>