Add support for documenting a portion of a request or response payload

Closes gh-312

docs/src/test/java/com/example/Payload.java

@@ -18,7 +18,9 @@ package com.example;
 
 import org.springframework.restdocs.payload.FieldDescriptor;
 
+import static org.springframework.restdocs.payload.PayloadDocumentation.beneathPath;
 import static org.springframework.restdocs.payload.PayloadDocumentation.fieldWithPath;
+import static org.springframework.restdocs.payload.PayloadDocumentation.responseFields;
 
 public class Payload {
 
@@ -31,4 +33,12 @@ public class Payload {
 		// end::book-descriptors[]
 	}
 
+	public void customSubsectionId() {
+		// tag::custom-subsection-id[]
+		responseFields(beneathPath("weather.temperature").withSubsectionId("temp"),
+				fieldWithPath("high").description("…"),
+				fieldWithPath("low").description("…"));
+		// end::custom-subsection-id[]
+	}
+
 }