Sysbuild: Documentation about namespace variables

[vChavezB]

Is your enhancement proposal related to a problem? Please describe.

I previously reported a bug with sysbuild due to not being able of reading CMake vars passed to the image. The problem resulted because I did not know that you need to call zephyr_get. Alas this is not documented in https://docs.zephyrproject.org/latest/build/sysbuild/index.html

Describe the solution you'd like

Document that zephyr_get cmake function should be called.