tidy up BCVInternalApi (add @MustBeDocumented)

Author: aSemy

modules/bcv-gradle-plugin/src/main/kotlin/internal/BCVInternalApi.kt

@@ -1,17 +1,20 @@
 package dev.adamko.kotlin.binary_compatibility_validator.internal
 
+import kotlin.RequiresOptIn.Level.WARNING
+import kotlin.annotation.AnnotationRetention.BINARY
 import kotlin.annotation.AnnotationTarget.*
 
 
 @RequiresOptIn(
   "Internal API - may change at any time without notice",
-  level = RequiresOptIn.Level.WARNING
+  level = WARNING
 )
-@Retention(AnnotationRetention.BINARY)
+@Retention(BINARY)
 @Target(
   CLASS,
   FUNCTION,
   PROPERTY,
   CONSTRUCTOR,
 )
+@MustBeDocumented
 internal annotation class BCVInternalApi