DOC: Clarify rank documentation (pandas-dev#40016)

Author: ptype

pandas/core/generic.py

@@ -8315,8 +8315,8 @@ def rank(
             How to rank NaN values:
 
             * keep: assign NaN rank to NaN values
-            * top: assign smallest rank to NaN values if ascending
-            * bottom: assign highest rank to NaN values if ascending.
+            * top: assign lowest rank to NaN values
+            * bottom: assign highest rank to NaN values
 
         ascending : bool, default True
             Whether or not the elements should be ranked in ascending order.