ESQL: Fix CASE when conditions are multivalued (#112401)

When CASE hits a multivalued field it was previously either crashing on
fold or evaluating it to the first value. Since booleans are loaded in
sorted order from lucene that *usually* means `false`. This changes the
behavior to line up with the rest of ESQL - now multivalued fields are
treated as `false` with a warning.

You might say "hey wait! multivalued fields usually become `null`, not
`false`!". Yes, dear reader, you are right. Very right. But! `CASE`'s
contract is to immediatly convert its values into `true` or `false`
using the standard boolean tri-valued logic. So `null` just become
`false` immediately. This is how PostgreSQL, MySQL, and SQLite behave:

```
> SELECT CASE WHEN null THEN 1 ELSE 2 END;
2
```

They turn that `null` into a false. And we're right there with them.
Except, of course, that we're turning `[false, false]` and the like into
`null` first. See!? It's consitent. Consistently confusing, but sane at
least.

The warning message just says "treating multivalued field as false"
rather than explaining all of that.

This also fixes up a few of CASE's docs which I noticed were kind of
busted while working on CASE. I think the docs generation is having a
lot of trouble with CASE so I've manually hacked the right thing into
place, but we should figure out a better solution eventually.

Closes #112359

Author: nik9000

docs/changelog/112401.yaml

@@ -0,0 +1,6 @@
+pr: 112401
+summary: "ESQL: Fix CASE when conditions are multivalued"
+area: ES|QL
+type: bug
+issues:
+ - 112359