Make it easier to document common portions of req and resp payloads

This commit adds a new andWithPrefix(String, FieldDescriptor[]) method
to both RequestFieldsSnippet and ResponseFieldsSnippet. It can be
used to add descriptors to an existing snippet, applying the given
prefix to the additional descriptors as it does so. This allows the
descriptors for a portion of a payload to be created once and then
reused, irrespective of where in the payload the portion appears.

Closes gh-221

spring-restdocs-core/src/test/java/org/springframework/restdocs/payload/RequestFieldsSnippetTests.java

@@ -176,4 +176,19 @@ public class RequestFieldsSnippetTests extends AbstractSnippetTests {
 						.content("{\"a\": {\"b\": 5, \"c\": \"charlie\"}}").build());
 	}
 
+	@Test
+	public void prefixedAdditionalDescriptors() throws IOException {
+		this.snippet.expectRequestFields("prefixed-additional-descriptors")
+				.withContents(tableWithHeader("Path", "Type", "Description")
+						.row("a", "Object", "one").row("a.b", "Number", "two")
+						.row("a.c", "String", "three"));
+
+		PayloadDocumentation.requestFields(fieldWithPath("a").description("one"))
+				.andWithPrefix("a.", fieldWithPath("b").description("two"),
+						fieldWithPath("c").description("three"))
+				.document(operationBuilder("prefixed-additional-descriptors")
+						.request("http://localhost")
+						.content("{\"a\": {\"b\": 5, \"c\": \"charlie\"}}").build());
+	}
+
 }

spring-restdocs-core/src/test/java/org/springframework/restdocs/payload/ResponseFieldsSnippetTests.java

@@ -232,4 +232,18 @@ public class ResponseFieldsSnippetTests extends AbstractSnippetTests {
 						.build());
 	}
 
+	@Test
+	public void prefixedAdditionalDescriptors() throws IOException {
+		this.snippet.expectResponseFields("prefixed-additional-descriptors")
+				.withContents(tableWithHeader("Path", "Type", "Description")
+						.row("a", "Object", "one").row("a.b", "Number", "two")
+						.row("a.c", "String", "three"));
+
+		PayloadDocumentation.responseFields(fieldWithPath("a").description("one"))
+				.andWithPrefix("a.", fieldWithPath("b").description("two"),
+						fieldWithPath("c").description("three"))
+				.document(operationBuilder("prefixed-additional-descriptors").response()
+						.content("{\"a\": {\"b\": 5, \"c\": \"charlie\"}}").build());
+	}
+
 }