(phan) Fix Phan warning

includes/ParagraphExtractor.php

@@ -290,10 +290,11 @@ public function findSnippet( $textToFind ) {
 	 * @param string[] $words Full prompt, e.g. [ "Sentence", "consists", "of", "words." ].
 	 * @param string[] $paragraphs Paragraphs to be searched.
 	 * @param string $oldQuery Previously found string, e.g. "Sequential words can form a".
-	 * @return mixed Search result. If not null ("not found"), contains the following keys:
+	 * @return ?array Search result. If not null ("not found"), contains the following keys:
 	 * string query Longest sequence of words from "words" array that have been found.
 	 * string[] paragraphs Paragraphs where "query" was found.
 	 * string[] leftoverWords Remaining words from "words" array that haven't been found yet.
+	 * @phan-return ?array{query:string,paragraphs:non-empty-associative-array<string>,leftoverWords:string[]}
 	 */
 	protected function findWordsRecursive( array $words, array $paragraphs, $oldQuery = '' ) {
 		if ( !$words ) {