update comments in groupby rank

Author: peterpanmj

pandas/_libs/groupby_helper.pxi.in

@@ -520,6 +520,8 @@ def group_rank_{{name}}(ndarray[float64_t, ndim=2] out,
                 grp_na_count += 1
                 out[_as[i], 0] = nan
                 grp_vals_seen = i + 1
+                # when label transition happens, update grp_size to keep track
+                # of number of nans encountered and increment grp_tie_count
                 if (i== N-1) or (labels[_as[i]] != labels[_as[i+1]]):
                     grp_tie_count +=1
                     if tiebreak != TIEBREAK_DENSE:
@@ -537,6 +539,11 @@ def group_rank_{{name}}(ndarray[float64_t, ndim=2] out,
                     grp_vals_seen = 1
                 continue
 
+            # Update out only when there is a transition of values or labels.
+            # When a new value or group is encountered, go back #dups steps(
+            # the number of occurrence of current value) and assign the ranks
+            # based on the the starting index of the current group (grp_start)
+            # and the current index
             if (i == N - 1 or
                     (masked_vals[_as[i]] != masked_vals[_as[i+1]]) or
                     (mask[_as[i]] ^ mask[_as[i+1]]) or
@@ -563,7 +570,7 @@ def group_rank_{{name}}(ndarray[float64_t, ndim=2] out,
                 # look forward to the next value (using the sorting in _as)
                 # if the value does not equal the current value then we need to
                 # reset the dups and sum_ranks, knowing that a new value is
-                # coming up. the conditional also needs to handle nan equality
+                # coming up. the condition also needs to handle nan equality
                 # and the end of iteration
                 if (i == N - 1 or
                         (masked_vals[_as[i]] != masked_vals[_as[i+1]]) or