doc: describe MSEL settings for booting from SD card

unmatched.rst describes booting from SD card or from SPI. But only for
booting from SPI the boot selection settings are described.

Add the missing information.

Fix a typo 'uSD'.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>