HADOOP-18216. io.file.buffer.size must be positive (#4220)

Documents that the buffer size must be > 0

Contributed by Jingxuan Fu.

hadoop-common-project/hadoop-common/src/main/resources/core-default.xml

@@ -793,7 +793,8 @@
   <description>The size of buffer for use in sequence files.
   The size of this buffer should probably be a multiple of hardware
   page size (4096 on Intel x86), and it determines how much data is
-  buffered during read and write operations.</description>
+  buffered during read and write operations. Must be greater than zero.
+  </description>
 </property>
 
 <property>