Documentation: How to update Zephyr itself (with west)

[dueringa]

Current state
Currently, the "Getting started Guide" only mentions the initial fetch of Zephyr with west init / west update. The "Beyond the Getting Started Guide" briefly mentions that

The west update command fetches and keeps Modules (External projects) in the zephyrproject folder in sync with the code in the local zephyr repo.
Warning
You must run west update any time the zephyr/west.yml changes, caused, for example, when you pull the zephyr repository, switch branches in it, or perform a git bisect inside of it.

Suggestion
I'd actually like the documentation making it more explicit "How to update Zephyr". Something along the lines of

Running west update doesn't actually update Zephyr. It only keeps the modules in a known state (as specified by the West manifest file.)
To actually update Zephyr.
cd <base>/zephyrproject/zephyr, followed by a git pull.
After that, run west update [...]

[carlescufi]

@dueringa perhaps you would consider sending a Pull Request against the corresponding .rst files in doc/?

[dueringa]

@carlescufi I can do that. Just wanted to check if my understanding was correct, first