The block diagram is missing that USART TX is double-buffered from Figure 25-1 in the data sheet. The figure below shows the added TX Buffer.

Figure 1. Block Diagram

The following text is changed in the Overview section:

The transmitter consists of a two-level write buffer.

The following text is changed in the Data Transmission section:

The data transmission is initiated by loading the Transmit Data (USARTn.TXDATAL and USARTn.TXDATAH) registers with the data to be sent. The data in the Transmit Data registers are moved to the TX Buffer once emptied and then to the Shift register once it is empty and ready to send a new frame.