[doc] RETENTION examples: example 4 references a non-existent column; example 3 expected output contradicts its INSERT

Verifying the `RETENTION` aggregate-function page (`sql-manual/sql-functions/aggregate-functions/retention.md`) on a live cluster (master daily; same text in version-3.x / version-4.x) surfaced two doc bugs. The page creates its own tables, so these are content errors, not missing setup.

**Bug 1 — example 4 queries a column that doesn't exist (errors instead of returning NULL)**

```sql
SELECT RETENTION(date = '2022-10-12') AS r FROM retention_test2 where uid is NULL;
```
`retention_test2` is defined as `(uid int, flag boolean, flag2 boolean)` — there is no `date` column, so this fails with:
`errCode = 2, detailMessage = Unknown column 'date' in 'table list'`.
The example's intent ("empty aggregation returns NULL") would be shown correctly with a column that exists, e.g. `SELECT RETENTION(flag) AS r FROM retention_test2 WHERE uid IS NULL;` (the `WHERE uid IS NULL` already filters to zero rows → NULL).

**Bug 2 — example 3 printed output contradicts its own INSERT**

```sql
INSERT into retention_test2 values (0, false, false), (1, true, NULL);
SELECT * from retention_test2;
```
The doc prints:
```
| uid | flag | flag2 |
| 0 | 1 | NULL |
| 1 | 0 | 0 |
```
but the inserted rows are `(0, false, false)` and `(1, true, NULL)`, i.e. the actual result is `(0, 0, 0)` and `(1, 1, NULL)`. The printed flag/flag2 values don't match the INSERT (they look transposed). The expected output should be corrected to match the data.

Both reproduce on a fresh single-BE cluster. Happy to send a PR correcting example 4's column and example 3's expected output if that direction is agreed.