Refresh the doc comments that describe an absent variable as null (#4076)

### Motivation

Follow-up to #4043, from @rickvdl's
[review](https://github.com/RevenueCat/purchases-android/pull/4043#pullrequestreview-5015541720).
Some doc comments were untrue after #4043.

### Description

The advice each comment gives is unchanged; only the reason behind it
moves.

- `CaseOperators` kept an example that can no longer happen, since the
lookup throws before `rc.lower` runs. Restated against an explicit
`null`, matching the wording iOS already merged.
- `RulesDimensionValue` also predates `rc.rootVar` (#4049), so its claim
that an iteration predicate cannot see the surrounding scope needed
narrowing to `var`.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Comment-only changes with no runtime or API impact.
> 
> **Overview**
> **Documentation-only follow-up** after #4043: comments still described
missing dimensions as resolving to `null` in the engine; behavior is now
an **unresolved variable** failure.
> 
> Updates KDoc in `DeviceDimensionProvider`, `RulesDimensionProvider`,
`RulesDimensionValue`, and `LocalRulesEvaluator` so omitted dimensions
and unevaluable predicates are described consistently—absent keys fail
resolution rather than silently non-matching, and unknown dimensions are
grouped with other predicates the SDK cannot evaluate when explaining
first-failure vs. later-rule-wins behavior.
> 
> `RulesDimensionValue.ObjectListValue` also notes that iteration
predicates can reach outer scope via **`rc.rootVar`** (#4049), not only
via fields on each record.
> 
> In `CaseOperators`, the no-coercion example is restated: explicit
`null` would stringify to `"null"` under `rc.lower`/`rc.upper`,
replacing an obsolete missing-`var` example that no longer applies after
lookup throws first.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
396914cedbe460a35a1d26b706aa1d9637a72eaa. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

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Co-authored-by: Cursor <cursoragent@cursor.com>