Improve explanation of top_chain_var parameter in Dynamic Topic Model…

… (DTM) documentation

* improve & corrected gensim documentation (piskvorky#2637)

* more descriptive explanation of top_chain_var

gensim/models/wrappers/dtmmodel.py

@@ -97,8 +97,10 @@ def __init__(self, dtm_path, corpus=None, time_slices=None, mode='fit', model='d
              Max em optimization iterations in LDA.
         alpha : int, optional
             Hyperparameter that affects sparsity of the document-topics for the LDA models in each timeslice.
-        top_chain_var : int, optional
-            Hyperparameter that affects.
+        top_chain_var : float, optional
+            This hyperparameter controls one of the key aspect of topic evolution which is the speed at which
+            these topics evolve. A smaller top_chain_var leads to similar word distributions over multiple timeslice.
+
         rng_seed : int, optional
              Random seed.
         initialize_lda : bool, optional