Some doc issues (#1772)

* Fix doxygen for uart example

Fixes https://github.com/raspberrypi/pico-sdk/issues/1685

* Clarify uart write

will block until data "has been sent to the UART transmit buffer"

Fixes https://github.com/raspberrypi/pico-sdk/issues/1481

* Add note to flash API about erasing sectors

Fixes: https://github.com/raspberrypi/pico-sdk/issues/650