Provide an API to ease documenting a request field’s constraints

Previously, users that wanted to document a request field’s constraints
had to roll their own solution. This commit introduces a new API that
makes it easier to document constraints. Support is provided for
discovering Bean Validation constraints and resolving descriptions for
them. The constraint descriptions can then be used as required. For
example, they can included in a field’s description or in an additional
constraints attribute that’s included in an additional table column via
the use of a custom snippet template.

Closes gh-42

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

@@ -0,0 +1,31 @@
+package com.example;
+
+import java.util.List;
+
+import javax.validation.constraints.NotNull;
+import javax.validation.constraints.Size;
+
+import org.springframework.restdocs.constraints.ConstraintDescriptions;
+
+public class Constraints {
+
+	@SuppressWarnings("unused")
+	// tag::constraints[]
+	public void example() {
+		ConstraintDescriptions userConstraints = new ConstraintDescriptions(UserInput.class); // <1>
+		List<String> descriptions = userConstraints.descriptionsForProperty("name"); // <2>
+	}
+	
+	static class UserInput {
+		
+		@NotNull
+		@Size(min = 1)
+		String name;
+		
+		@NotNull
+		@Size(min = 8)
+		String password;
+	}
+	// end::constraints[]
+	
+}