doc: flash_debug: fix some issues with probes documentation

Fix a few issues with the probes documentation for NXP boards:
- fix references to debug probe type in probes page introduction
- add an explicit "Install the host tools" step for the LPC-Link and
  MCU-Link based debug probe update steps.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>