boards: kconfig: Fix enabling USE_SEGGER_RTT for various boards

HAS_SEGGER_RTT is assigned by various Nordic boards, but assignments
have no effect on promptless symbols. This symbol is enabled through
being select'ed by SOC_SERIES_NRF52X.

Holyiot, nRF52833-PCA10100, nRF52840-PCA10056, and nRF52-PCA10040 only
assign HAS_SEGGER_RTT without assigning USE_SEGGER_RTT. They probably
meant to enable USE_SEGGER_RTT, so do that instead.

Also add a help text to HAS_SEGGER_RTT and a warning re. HAS_SEGGER_RTT
vs. USE_SEGGER_RTT to the documentation.

Flagged by https://github.com/zephyrproject-rtos/zephyr/pull/20742

.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>