APPROX_COUNT_DISTINCT no listed in aggregation functions

[0xdeafbeef]

APPROX_COUNT_DISTINCT() is mentioned in supported functions to pushdown to tiflash by not listed in

docs/functions-and-operators/aggregate-group-by-functions.md

Line 4 in ec29eb8

aliases: ['/docs/dev/functions-and-operators/aggregate-group-by-functions/','/docs/dev/reference/sql/functions-and-operators/aggregate-group-by-functions/']

docs/tiflash/tiflash-supported-pushdown-calculations.md

Line 47 in ec29eb8

| [Aggregate functions](/functions-and-operators/aggregate-group-by-functions.md) | `MIN()`, `MAX()`, `SUM()`, `COUNT()`, `AVG()`, `APPROX_COUNT_DISTINCT()`, `GROUP_CONCAT()` |