Add scaladocs

core/src/main/scala/com/evolutiongaming/kafka/flow/EnhancedFold.scala

@@ -26,13 +26,19 @@ trait EnhancedFold[F[_], S, E] {
       }
     }
 
+  /**
+    * Returns a fold that discards events for which f returns false
+    */
   final def filter(f: (S, E) => Boolean)(implicit F: Applicative[F]): EnhancedFold[F, S, E] =
     (extras, state0, event) =>
       state0 match {
         case Some(state) => if (f(state, event)) apply(extras, state0, event) else F.pure(state0)
         case None        => apply(extras, state0, event)
       }
 
+  /**
+    * Similar to `filter`
+    */
   final def filterM(f: (S, E) => F[Boolean])(implicit F: Monad[F]): EnhancedFold[F, S, E] =
     (extras, state0, event) =>
       state0 match {

core/src/main/scala/com/evolutiongaming/kafka/flow/Fold.scala

@@ -6,7 +6,10 @@ import cats.Functor
 import cats.Monad
 import cats.syntax.all._
 
-/** Reads a state and effectfully produces a new one.
+/**
+  * @see [[com.evolutiongaming.kafka.flow.EnhancedFold]]
+  *
+  * Reads a state and effectfully produces a new one.
   *
   * Roughly speaking it is `Kleisli[F, (S, A), S]` with the main additional
   * requirement that input and output types are not independent (because `S`

core/src/main/scala/com/evolutiongaming/kafka/flow/FoldOption.scala

@@ -6,7 +6,11 @@ import cats.Functor
 import cats.Monad
 import cats.syntax.all._
 
-/** Convenience methods for using `Fold` with optional state */
+/**
+  * @see [[com.evolutiongaming.kafka.flow.EnhancedFold]]
+  *
+  * Convenience methods for using `Fold` with optional state
+  * */
 final case class FoldOption[F[_], S, A](value: Fold[F, Option[S], A]) {
 
   /** Alias for `run` */

core/src/main/scala/com/evolutiongaming/kafka/flow/FoldToState.scala

@@ -8,7 +8,10 @@ import cats.syntax.all._
 import com.evolutiongaming.kafka.flow.effect.CatsEffectMtlInstances._
 import com.evolutiongaming.kafka.flow.persistence.Persistence
 
-/** Applies records to a state stored inside and informs the listeners about the changes */
+/**
+  * Applies records to a state stored inside and performs the necessary updates,
+  * for example, it can persist the state and remove it from memory.
+  */
 trait FoldToState[F[_], E] {
 
   def apply(records: NonEmptyList[E]): F[Unit]

core/src/main/scala/com/evolutiongaming/kafka/flow/KeyFlowOf.scala

@@ -6,6 +6,9 @@ import com.evolutiongaming.kafka.flow.persistence.Persistence
 import com.evolutiongaming.kafka.flow.registry.EntityRegistry
 import com.evolutiongaming.kafka.flow.timer.{TimerContext, TimerFlowOf}
 
+/**
+  * Factory of KeyFlows
+  */
 trait KeyFlowOf[F[_], S, A] {
 
   def apply(

core/src/main/scala/com/evolutiongaming/kafka/flow/KeyStateOf.scala

@@ -11,6 +11,9 @@ import com.evolutiongaming.kafka.journal.ConsRecord
 import com.evolutiongaming.skafka.TopicPartition
 import com.evolutiongaming.sstream.Stream
 
+/**
+  * Factory of KeyStates
+  */
 trait KeyStateOf[F[_]] { self =>
 
   /** Creates or restores a state for a single key */

core/src/main/scala/com/evolutiongaming/kafka/flow/PartitionFlowOf.scala

@@ -7,6 +7,9 @@ import com.evolutiongaming.kafka.flow.PartitionFlow.FilterRecord
 import com.evolutiongaming.kafka.flow.kafka.ScheduleCommit
 import com.evolutiongaming.skafka.{Offset, TopicPartition}
 
+/**
+  * Factory of PartitionFlows
+  */
 trait PartitionFlowOf[F[_]] {
 
   /** Creates partition record handler for assigned partition */

core/src/main/scala/com/evolutiongaming/kafka/flow/Tick.scala

@@ -4,6 +4,11 @@ import cats.Applicative
 import cats.syntax.all._
 import cats.Functor
 
+/**
+  * Updates an aggregate. Tick is similar to [[com.evolutiongaming.kafka.flow.EnhancedFold]],
+  * but gets triggered by timers instead of incoming events.
+  * For example, you may want to regularly delete aggregates that haven't been updated for N minutes.
+  */
 case class Tick[F[_], S](run: S => F[S]) {
 
   /** Alias for `run` */

core/src/main/scala/com/evolutiongaming/kafka/flow/TickOption.scala

@@ -3,6 +3,9 @@ package com.evolutiongaming.kafka.flow
 import cats.Applicative
 import cats.Functor
 
+/**
+  * Similar to [[com.evolutiongaming.kafka.flow.Tick]]
+  */
 case class TickOption[F[_], S](value: Tick[F, Option[S]]) {
 
   /** Alias for `value.run` */

core/src/main/scala/com/evolutiongaming/kafka/flow/TopicFlowOf.scala

@@ -6,6 +6,9 @@ import com.evolutiongaming.catshelper.{LogOf, Runtime}
 import com.evolutiongaming.kafka.flow.kafka.Consumer
 import com.evolutiongaming.skafka.Topic
 
+/**
+  * Factory of TopicFlows
+  */
 trait TopicFlowOf[F[_]] {
 
   def apply(consumer: Consumer[F], topic: Topic): Resource[F, TopicFlow[F]]

core/src/main/scala/com/evolutiongaming/kafka/flow/timer/TimerFlowOf.scala

@@ -9,6 +9,9 @@ import com.evolutiongaming.kafka.flow.persistence.FlushBuffers
 
 import scala.concurrent.duration._
 
+/**
+  * Factory of TimerFlows
+  */
 trait TimerFlowOf[F[_]] {
 
   def apply(

core/src/main/scala/com/evolutiongaming/kafka/flow/timer/TimersOf.scala

@@ -4,6 +4,9 @@ import cats.effect.Sync
 import cats.syntax.all._
 import com.evolutiongaming.catshelper.LogOf
 
+/**
+  * Factory of TimerContexts
+  */
 trait TimersOf[F[_], K] {
 
   def apply(key: K, createdAt: Timestamp): F[TimerContext[F]]