Update code comment to reflect the range of IF from [1, k] (#4139)

Summary: Pull Request resolved: #4139

Reviewed By: junjieqi

Differential Revision: D68630805

fbshipit-source-id: ff2446e8179cefcf588b58673239ef8549d9fd1d

Author: Amir Sadoughi

faiss/utils/utils.h

@@ -90,7 +90,8 @@ size_t merge_result_table_with(
         bool keep_min = true,
         int64_t translation = 0);
 
-/// a balanced assignment has a IF of 1
+/// a balanced assignment has a IF of 1, a completely unbalanced assignment has
+/// an IF = k.
 double imbalance_factor(int n, int k, const int64_t* assign);
 
 /// same, takes a histogram as input