[LLC, Persistence] Add doc comments

Signed-off-by: Sahil Kumar <[email protected]>

packages/stream_chat_persistence/lib/src/converter/list_converter.dart

@@ -2,7 +2,8 @@ import 'dart:convert';
 
 import 'package:moor/moor.dart';
 
-///
+/// Maps a [List] of type [T] into a [String] understood
+/// by the sqlite backend.
 class ListConverter<T> extends TypeConverter<List<T>, String> {
   @override
   List<T> mapToDart(fromDb) {

packages/stream_chat_persistence/lib/src/converter/map_converter.dart

@@ -2,7 +2,8 @@ import 'dart:convert';
 
 import 'package:moor/moor.dart';
 
-///
+/// Maps a [Map] of type [String], [T] into a [String] understood
+/// by the sqlite backend.
 class MapConverter<T> extends TypeConverter<Map<String, T>, String> {
   @override
   Map<String, T> mapToDart(fromDb) {

packages/stream_chat_persistence/lib/src/converter/message_sending_status_converter.dart

@@ -1,7 +1,8 @@
 import 'package:moor/moor.dart';
 import 'package:stream_chat/stream_chat.dart';
 
-///
+/// Maps a [MessageSendingStatus] into a [int] understood
+/// by the sqlite backend.
 class MessageSendingStatusConverter
     extends TypeConverter<MessageSendingStatus, int> {
   @override