add docs for PinChatMessage

tgbotapi.core/src/commonMain/kotlin/dev/inmo/tgbotapi/requests/chat/modify/PinChatMessage.kt

@@ -6,6 +6,11 @@ import dev.inmo.tgbotapi.types.*
 import kotlinx.serialization.*
 import kotlinx.serialization.builtins.serializer
 
+/**
+ * Use this method to add a message to the list of pinned messages in a chat. If the chat is not a private chat, the bot
+ * must be an administrator in the chat for this to work and must have the 'can_pin_messages' admin right in a
+ * supergroup or 'can_edit_messages' admin right in a channel.
+ */
 @Serializable
 data class PinChatMessage (
     @SerialName(chatIdField)