Update javadoc to be Java 11 compatible

Closes gh-687

build.gradle

@@ -155,6 +155,7 @@ configure(publishedCodeProjects) { subproject ->
 		options.links = javadocLinks
 		options.addStringOption '-quiet'
 		options.encoding = 'UTF-8'
+		options.source = '1.8'
 	}
 
 	task sourcesJar(type: Jar) {

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/PayloadDocumentation.java

@@ -68,7 +68,8 @@ public abstract class PayloadDocumentation {
 	 *
 	 * The following paths are all present:
 	 *
-	 * <table summary="Paths and their values">
+	 * <table>
+	 * <caption>Paths that are present and their values</caption>
 	 * <tr>
 	 * <th>Path</th>
 	 * <th>Value</th>
@@ -132,7 +133,8 @@ public abstract class PayloadDocumentation {
 	 *
 	 * The following paths are all present:
 	 *
-	 * <table summary="Paths and their values">
+	 * <table>
+	 * <caption>Paths that are present and their values</caption>
 	 * <tr>
 	 * <th>Path</th>
 	 * <th>Value</th>