docs: clarify distinct kwarg on sum and avg

Drop the unhelpful "upstream avg_distinct/sum_distinct shortcut"
reference in favor of describing the actual behavior.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

python/datafusion/functions.py

@@ -4533,8 +4533,7 @@ def avg(
 
     Args:
         expression: Values to combine into an array
-        distinct: If True, only distinct values are averaged. Equivalent to the
-            upstream ``avg_distinct`` shortcut.
+        distinct: If True, duplicate values are removed before averaging.
         filter: If provided, only compute against rows for which the filter is True
 
     Examples:
@@ -4861,8 +4860,7 @@ def sum(
 
     Args:
         expression: Values to combine into an array
-        distinct: If True, only distinct values are summed. Equivalent to the
-            upstream ``sum_distinct`` shortcut.
+        distinct: If True, duplicate values are removed before summing.
         filter: If provided, only compute against rows for which the filter is True
 
     Examples: