add kdoc for Post.kt

core/src/main/kotlin/com/insanusmokrassar/postssystem/core/post/Post.kt

@@ -9,10 +9,17 @@ typealias PostId = String
 typealias PostContents = List<Content>
 typealias RegisteredPostContents = List<RegisteredContent>
 
+/**
+ * Base interface for creating of new post. Usually, it is just [SimplePost] instance
+ */
 interface Post {
     val content: PostContents
 }
 
+/**
+ * Root entity of whole system. Can be retrieved from [com.insanusmokrassar.postssystem.core.api.ReadPostsAPI] by
+ * getting and created in [com.insanusmokrassar.postssystem.core.api.WritePostsAPI] by inserting of [Post] instance
+ */
 interface RegisteredPost : Post {
     val id: PostId
 
@@ -22,10 +29,17 @@ interface RegisteredPost : Post {
     val modificationDate: DateTime
 }
 
+/**
+ * Base and currently (1st Nov 2019) single realisation of [Post]. There is [SimpleRegisteredPost] which is technically
+ * is [Post] too, but it is not direct [Post] realisation
+ */
 data class SimplePost(
     override val content: PostContents
 ) : Post
 
+/**
+ * Base and currently (1st Nov 2019) single realisation of [RegisteredPost]
+ */
 data class SimpleRegisteredPost(
     override val id: PostId,
     override val content: RegisteredPostContents,