HBASE-17086: Add comments to explain why Cell#getTagsLength() returns an int, rather than a short

Signed-off-by: anoopsamjohn <anoopsamjohn@gmail.com>

hbase-common/src/main/java/org/apache/hadoop/hbase/Cell.java

@@ -170,6 +170,7 @@ public interface Cell {
   int getValueLength();
 
   /**
+   * Contiguous raw bytes representing tags that may start at any index in the containing array.
    * @return the tags byte array
    */
   byte[] getTagsArray();
@@ -180,6 +181,12 @@ public interface Cell {
   int getTagsOffset();
 
   /**
+   * HBase internally uses 2 bytes to store tags length in Cell.
+   * As the tags length is always a non-negative number, to make good use of the sign bit,
+   * the max of tags length is defined as {@link TagUtil#MAX_TAGS_LENGTH}, which is 2 * Short.MAX_VALUE + 1 = 65535.
+   * As a result, the return type is int, because a short is not capable of handling that.
+   * Please note that even if the return type is int, the max tags length is far less than Integer.MAX_VALUE.
+   *
    * @return the total length of the tags in the Cell.
    */
   int getTagsLength();