Improves documentation and usage (#2038)

Author: nedtwigg

plugin-gradle/CHANGES.md

@@ -138,7 +138,7 @@ We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (
   ```
   spotless {
     java {
-      eclipse().withP2Mirrors(['https://download.eclipse.org/', 'https://some.internal.mirror/eclipse'])
+      eclipse().withP2Mirrors(['https://download.eclipse.org/': 'https://some.internal.mirror/eclipse'])
     }
   }
   ```

plugin-gradle/README.md

@@ -234,6 +234,10 @@ spotless {
     eclipse()
     // optional: you can specify a specific version and/or config file
     eclipse('4.26').configFile('eclipse-prefs.xml')
+    // if the access to the p2 repositories is restricted, mirrors can be
+    // specified using a URI prefix map as follows:
+    echlise().withP2Mirrors(['https://download.eclipse.org/eclipse/updates/4.29/':'https://some.internal.mirror/4-29-updates-p2/'])
+       
 ```
 
 

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/BaseGroovyExtension.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023 DiffPlug
+ * Copyright 2023-2024 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -64,11 +64,12 @@ private GrEclipseConfig(String version, FormatExtension extension) {
 			extension.addStep(builder.build());
 		}
 
-		public void configFile(Object... configFiles) {
+		public GrEclipseConfig configFile(Object... configFiles) {
 			requireElementsNonNull(configFiles);
 			Project project = extension.getProject();
 			builder.setPreferences(project.files(configFiles).getFiles());
 			extension.replaceStep(builder.build());
+			return this;
 		}
 
 		public GrEclipseConfig withP2Mirrors(Map<String, String> mirrors) {

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/CppExtension.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2023 DiffPlug
+ * Copyright 2016-2024 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -52,11 +52,12 @@ public class EclipseConfig {
 			addStep(builder.build());
 		}
 
-		public void configFile(Object... configFiles) {
+		public EclipseConfig configFile(Object... configFiles) {
 			requireElementsNonNull(configFiles);
 			Project project = getProject();
 			builder.setPreferences(project.files(configFiles).getFiles());
 			replaceStep(builder.build());
+			return this;
 		}
 
 		public EclipseConfig withP2Mirrors(Map<String, String> mirrors) {