samples/doc: segger: rtt: enhanced tracing/logging

Fixes a minor documentation issue and suggests/documents additional RTT
tracing and logging options.

Showcases logging via RTT UART which is now available w/o restrictions
due to the preceding change. This has been tested on CC13/26xx hardware.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>