Claude review: kunit: Add documentation for warning backtrace suppression API

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Overall: Clear and correct documentation.**

The documentation covers both API forms with code examples and includes the important note about checking warning counts inside the scoped block.

**Minor: The doc shows `kunit_suppressed_warning_count(w)` used after `kunit_end_suppress_warning()`:**

```rst
+		w = kunit_start_suppress_warning(test);
+		trigger_backtrace();
+		kunit_end_suppress_warning(test, w);
+
+		KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
```

This is valid because `w` remains allocated (via `kunit_kzalloc`) until test exit. But it's worth being explicit in the documentation that `w` remains readable after `kunit_end_suppress_warning` — a reader might assume end-suppress frees the handle. The test code in patch 2 (`backtrace_suppression_test_multi_scope`) relies on this same property, so it's an intentional design choice. A one-line note in the docs like "The handle remains valid for count queries after ``kunit_end_suppress_warning()``" would prevent confusion.

**Fixes missing newline at EOF:**

```diff
-	}
\ No newline at end of file
+	}
```

Nice cleanup.

**Summary:** This is a mature, well-reviewed series at v11. The core design is sound, the test coverage is thorough, and the DRM usage patch is a good practical demonstration. The issues identified are all minor style/documentation nits rather than correctness problems. From the DRM subsystem perspective, patch 3 is a clean improvement to the drm_rect tests.

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