Add scaladoc for EventualPayloadAndType and related classes (#557)

core/src/main/scala/com/evolutiongaming/kafka/journal/eventual/EventualPayloadAndType.scala

@@ -7,6 +7,20 @@ import com.evolutiongaming.kafka.journal.util.Fail
 import com.evolutiongaming.kafka.journal.util.Fail.implicits._
 import scodec.bits.ByteVector
 
+/** Piece of data prepared for convenient storing into Cassandra row.
+  *
+  * Usually the data is stored into the structure similar to the following:
+  * {{{
+  * payload_type TEXT,
+  * payload_txt TEXT,
+  * payload_bin BLOB
+  * }}}
+  * Where usage of either `payload_txt` or `payload_bin` column depends on
+  * the contents of a `payload` field.
+  *
+  * The `payloadType` field is used to determine how the contents of `payload`
+  * should be treated, i.e. if it should be parsed as JSON.
+  */
 final case class EventualPayloadAndType(
   payload: Either[String, ByteVector],
   payloadType: PayloadType

core/src/main/scala/com/evolutiongaming/kafka/journal/eventual/EventualRead.scala

@@ -8,6 +8,11 @@ import com.evolutiongaming.kafka.journal.util.Fail.implicits._
 import com.evolutiongaming.kafka.journal.{JournalError, JsonCodec, Payload, PayloadType}
 
 
+/** Decode a payload loaded from an eventual storage.
+  *
+  * Converts a structure convenient to store to eventual store, i.e. Cassandra, to a structure, which is convenient to
+  * use for a business logic.
+  */
 trait EventualRead[F[_], A] {
 
   def apply(payloadAndType: EventualPayloadAndType): F[A]

core/src/main/scala/com/evolutiongaming/kafka/journal/eventual/EventualWrite.scala

@@ -6,6 +6,11 @@ import com.evolutiongaming.kafka.journal.{JournalError, JsonCodec, Payload}
 import scodec.bits.ByteVector
 
 
+/** Prepare payload for storing.
+  *
+  * Converts a structure convenient for a business logic to a structure, which is convenient to store to eventual store,
+  * i.e. Cassandra.
+  */
 trait EventualWrite[F[_], A] {
 
   def apply(payload: A): F[EventualPayloadAndType]