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

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

Patch Review

**Good documentation** covering both API forms with clear examples. The RST formatting looks correct.

**One minor doc issue:** The documentation shows the direct API example checking the count *after* `kunit_end_suppress_warning()`:

```c
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 works because the `kunit_kzalloc`'d memory outlives the suppression deactivation (as confirmed by the multi_scope selftest). But it's worth noting this is a subtlety that could trip up readers — the handle remains readable after `kunit_end_suppress_warning()` because the allocation is test-scoped. The doc might benefit from a brief note about this, but it's not a blocker.

**Fix at the end:** The patch also fixes a missing newline at end of file (`\ No newline at end of file` → proper newline). Good housekeeping.

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