add small docs for WritePostsAPI

core/src/main/kotlin/com/insanusmokrassar/postssystem/core/post/api/WritePostsAPI.kt

@@ -9,6 +9,10 @@ interface WritePostsAPI {
     val postDeletedFlow: Flow<RegisteredPost>
     val postUpdatedFlow: Flow<RegisteredPost>
 
+    /**
+     * For creating of post you need to create all its [com.insanusmokrassar.postssystem.core.content.RegisteredContent]
+     * and (or just) retrieve their [ContentIds] and put it into some [Post] implementation line [SimplePost]
+     */
     suspend fun createPost(post: Post): RegisteredPost?
     suspend fun deletePost(id: PostId): Boolean