uart.4: describe better + spdx

+ add serial to description (search keywords)
+ remove redundant (UART), for, and device
+ description is now one line on standard cons.

MFC after: 3 days
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1531

(cherry picked from commit db512bb30314326a0b2cfad3f9094dd830744777)