[Docs] IN_ID_SET

This is a note for me of something that needs documenting

```
xtrntr 1 day ago
will using IdSet with “NOT IN” clause have any unintended performance impact?
e.g. select * from table where userid not in IDSET(...)

Jackie 21 hours ago
This is not correct syntax. You should do select ... where IN_ID_SET(userid, ...) = 0
```

Start by locating Pinot’s existing documentation for IN_ID_SET and review the syntax shown in the issue. Document the correct form for checking non-membership and clarify any performance implications of using IN_ID_SET with a NOT IN-style condition.

Written by the indexing model from the issue text.