op-batcher: Add channelBuilder.InputBytes and improve docs

op-batcher/batcher/channel_builder.go

@@ -91,10 +91,19 @@ func (c *channelBuilder) ID() derive.ChannelID {
 	return c.co.ID()
 }
 
+// InputBytes returns to total amount of input bytes added to the channel.
+func (c *channelBuilder) InputBytes() int {
+	return c.co.InputBytes()
+}
+
+// Blocks returns a backup list of all blocks that were added to the channel. It
+// can be used in case the channel needs to be rebuilt.
 func (c *channelBuilder) Blocks() []*types.Block {
 	return c.blocks
 }
 
+// Reset resets the internal state of the channel builder so that it can be
+// reused. Note that a new channel id is also generated by Reset.
 func (c *channelBuilder) Reset() error {
 	c.blocks = c.blocks[:0]
 	c.frames = c.frames[:0]