docs(test): add KDoc to companion object

Fixes Codacy documentation warning.

AI-assisted: Claude Code (Sonnet 4.6)
Signed-off-by: Jan C. Borchardt <925062+jancborchardt@users.noreply.github.com>

Author: jancborchardt

app/src/test/java/it/niedermann/owncloud/notes/shared/util/NoteUtilTest.kt

@@ -21,6 +21,7 @@ import org.robolectric.annotation.Config
 @RunWith(RobolectricTestRunner::class)
 class NoteUtilTest : TestCase() {
 
+    /** Assertion messages shared across test methods. */
     companion object {
         private const val NO_RAW_HTML_TAGS = "Excerpts must not contain raw HTML tags"
         private const val TRUNCATED_TO_200 = "Excerpt must be truncated to 200 chars"