[doc] Fix the default value for lambdarank_pair_method. (#10098)

doc/parameter.rst

@@ -489,7 +489,7 @@ Parameters for learning to rank (``rank:ndcg``, ``rank:map``, ``rank:pairwise``)
 
 These are parameters specific to learning to rank task. See :doc:`Learning to Rank </tutorials/learning_to_rank>` for an in-depth explanation.
 
-* ``lambdarank_pair_method`` [default = ``mean``]
+* ``lambdarank_pair_method`` [default = ``topk``]
 
   How to construct pairs for pair-wise learning.
 
@@ -502,6 +502,8 @@ These are parameters specific to learning to rank task. See :doc:`Learning to Ra
 
 * ``lambdarank_normalization`` [default = ``true``]
 
+  .. versionadded:: 2.1.0
+
   Whether to normalize the leaf value by lambda gradient. This can sometimes stagnate the training progress.
 
 *  ``lambdarank_unbiased`` [default = ``false``]