docs/build-flash-debug: expand one-time CMake arguments section

- Change "re-run CMake" to "re-run CMake configuration steps" because
  `west build -v ...` displays `cmake --build ...` every time which can
  confuse non-experts.

- Explain what is the actual issue with re-configuring: it makes the
  incremental builds slower. But not much slower:

```
   $ west  build -b qemu_x86 samples/hello_world/ -DCONFIG_OUTPUT_STAT=y
   [133/133] Linking C executable zephyr/zephyr.elf
   $ west  build -b qemu_x86 samples/hello_world/ -DCONFIG_OUTPUT_STAT=y
   [16/16] Linking C executable zephyr/zephyr.elf
```

- Explain that the best solution is to simply skip to the next section

Signed-off-by: Marc Herbert <marc.herbert@intel.com>