add kdoc to SpecialContent and Content

core/src/main/kotlin/com/insanusmokrassar/postssystem/core/content/Content.kt

@@ -2,8 +2,16 @@ package com.insanusmokrassar.postssystem.core.content
 
 typealias ContentId = String
 
+/**
+ * Content which is planned to be registered in database
+ */
 interface Content
 
+/**
+ * Content which is already registered in database. Using its [id] you can retrieve all known
+ * [com.insanusmokrassar.postssystem.core.post.RegisteredPost]s by using
+ * [com.insanusmokrassar.postssystem.core.api.ReadPostsAPI.getPostsByContent]
+ */
 interface RegisteredContent : Content {
     val id: ContentId
 }

core/src/main/kotlin/com/insanusmokrassar/postssystem/core/content/SpecialContent.kt

@@ -2,10 +2,20 @@ package com.insanusmokrassar.postssystem.core.content
 
 import kotlinx.serialization.Serializable
 
+/**
+ * Special content, which usually better known in some plugin or system, but is not representable in others. For
+ * example, it could be issue in some git system, which is unknown for all other systems.
+ *
+ * @property internalId Represent Id of this content in system which knows exact type and can correctly handle this
+ * content
+ */
 interface SpecialContent : Content {
     val internalId: ContentId
 }
 
+/**
+ * As [SpecialContent], but already registered in posts system
+ */
 interface SpecialRegisteredContent : RegisteredContent, SpecialContent
 
 @Serializable