CLN: adding comments for when val is -1 (#35498)

Author: alexhlim

pandas/_libs/index.pyx

@@ -336,12 +336,16 @@ cdef class IndexEngine:
                         n_alloc += 10_000
                         result = np.resize(result, n_alloc)
 
+                    # -1 in targets could be either -1 or nan
+                    # ensures values in d[-1] to be included only once
                     if val == -1:
                         nan_val = values_mask[j]
+                        # nan
                         if nan_target:
                             if nan_val:
                                 result[count] = j
                                 count += 1
+                        # -1
                         else:
                             if not nan_val:
                                 result[count] = j