Update docs to ValidationPlugin and its implementation

Author: yevhenii-nadtochii

java/src/main/kotlin/io/spine/validation/java/JavaValidationPlugin.kt

@@ -30,11 +30,12 @@ import io.spine.validation.ValidationPlugin
 import io.spine.validation.java.setonce.SetOnceRenderer
 
 /**
- * A plugin that sets up everything needed to generate Java validation code.
+ * An implementation of [ValidationPlugin] for Java language.
  *
- * This plugin uses a delegate plugin to set up some of the components needed for
- * code generation. By default, a [ValidationPlugin] is used. However, API users may
- * extend this plugin's behavior and supply a more rich base plugin.
+ * Note: [`(set_once)`][SetOnceRenderer] option is rendered with its own
+ * renderer because it significantly differs from the rest of constraints.
+ * This option modifies the message builder behavior. The rest of
+ * the constraints are more about message state assertions.
  */
 @Suppress("unused") // Accessed via reflection.
 public class JavaValidationPlugin : ValidationPlugin(

model/src/main/kotlin/io/spine/validation/ValidationPlugin.kt

@@ -36,9 +36,14 @@ import io.spine.validation.required.RequiredIdPatternPolicy
 import io.spine.validation.required.RequiredPolicy
 
 /**
- * A ProtoData plugin which attaches validation-related policies and views.
+ * The basic implementation of ProtoData validation plugin, which builds
+ * language-agnostic representation of the declared constraints.
+ *
+ * The concrete implementations should provide [renderers], which will
+ * implement these constraints for a specific programming language.
+ * For example, Java, Dart, or TypeScript.
  */
-public open class ValidationPlugin(renderers: List<Renderer<*>> = emptyList()) : Plugin(
+public abstract class ValidationPlugin(renderers: List<Renderer<*>> = emptyList()) : Plugin(
     renderers = renderers,
     views = setOf(),
     viewRepositories = setOf<ViewRepository<*, *, *>>(

model/src/test/kotlin/io/spine/validation/PolicySpec.kt

@@ -87,7 +87,7 @@ class PolicySpec {
             ProtoFileList(emptyList()),
             emptySet()
         )
-        val plugin = ValidationPlugin()
+        val plugin = object : ValidationPlugin() {}
         codegenContext = CodeGenerationContext(Pipeline.generateId(), typeSystem) {
             // Mimic what a `Pipeline` does to its plugins.
             plugin.applyTo(this, typeSystem)

model/src/testFixtures/kotlin/io/spine/validation/ValidationTestFixture.kt

@@ -50,7 +50,7 @@ import java.nio.file.Path
 class ValidationTestFixture(
     descriptor: GenericDescriptor,
     workingDir: Path,
-    plugin: Plugin = ValidationPlugin()
+    plugin: Plugin = object : ValidationPlugin() {}
 ) {
     val setup: PipelineSetup