doc: west: improve troubleshooting for west update

   west.manifest: running 'git fetch ... https://github.com/your-username/your_project ...' in /some/directory

The ``git fetch`` command example in the last line above is what needs to

succeed. Go to ``/some/directory``, copy/paste and run the entire ``git fetch``

command, then debug from there using the documentation for your credential

storage helper.

succeed.

One strategy is to go to ``/some/directory``, copy/paste and run the entire

``git fetch`` command, then debug from there using the documentation for your

credential storage helper.

If you're behind a corporate firewall and may have proxy or other issues,

``curl -v FETCH_URL`` (for HTTPS URLs) or ``ssh -v FETCH_URL`` (for SSH URLs)

may be helpful.

If you can get the ``git fetch`` command to run successfully without prompting

for a password when you run it directly, you will be able to run ``west