Clarifying documentation in USART chapter. #422

Job	Run time
build-f3discovery-doc	57s
build-book	2m 4s
build-microbit-chapter (05-led-roulette)	34s
build-f3discovery-chapter (05-led-roulette)	34s
build-microbit-chapter (07-uart)	38s
build-microbit-doc	2m 9s
build-f3discovery-chapter (06-hello-world)	30s
build-f3discovery-chapter (07-registers)	26s
build-f3discovery-chapter (08-leds-again)	22s
build-f3discovery-chapter (09-clocks-and-timers)	29s
build-microbit-chapter (08-i2c)	31s
build-f3discovery-chapter (11-usart)	36s
build-microbit-chapter (09-led-compass)	36s
build-f3discovery-chapter (14-i2c)	28s
build-microbit-chapter (10-punch-o-meter)	36s
build-f3discovery-chapter (15-led-compass)	22s
build-f3discovery-chapter (16-punch-o-meter)	21s
	12m 13s

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