Document IAE in HttpHeaders methods that parse ETags

See gh-24950

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -1140,6 +1140,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 
 	/**
 	 * Return the value of the {@code If-Match} header.
+	 * @throws IllegalArgumentException if parsing fails
 	 * @since 4.3
 	 */
 	public List<String> getIfMatch() {
@@ -1199,6 +1200,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 
 	/**
 	 * Return the value of the {@code If-None-Match} header.
+	 * @throws IllegalArgumentException if parsing fails
 	 */
 	public List<String> getIfNoneMatch() {
 		return getETagValuesAsList(IF_NONE_MATCH);
@@ -1542,6 +1544,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 * Retrieve a combined result from the field values of the ETag header.
 	 * @param headerName the header name
 	 * @return the combined result
+	 * @throws IllegalArgumentException if parsing fails
 	 * @since 4.3
 	 */
 	protected List<String> getETagValuesAsList(String headerName) {