Remove unresolvable KDoc links

Author: alexander-yevsyukov

context/src/main/kotlin/io/spine/tools/validation/ErrorPlaceholder.kt

@@ -33,16 +33,15 @@ package io.spine.tools.validation
  * Each validation option declares the supported placeholders. Take a look at
  * `options.proto` for examples.
  *
- * Important Note: we have the same [enum][io.spine.validation.RuntimeErrorPlaceholder]
+ * ### Important Note
+ *
+ * we have the same items in this enum as in `io.spine.validation.RuntimeErrorPlaceholder`
  * in the runtime library, which is exactly as this one. Please keep them in sync.
  * This duplication is done intentionally to prevent clash between the runtime library,
  * which is added to the classpath of the Compiler and the runtime library, which is part
  * of the Compiler itself because it is a part of Spine. As we complete our migration
  * of validation to codegen, the runtime library will either be significantly simplified,
  * or even its content may be moved to `base`. Then, the duplicate enum should be removed.
- *
- * @see io.spine.validation.TemplateString
- * @see io.spine.validation.RuntimeErrorPlaceholder
  */
 public enum class ErrorPlaceholder(public val value: String) {