Improve documentation on how to get the serial path

[Drachenkaetzchen]

Right now, the documentation and all sample configs refer to using ls -l /dev/serial/by-id. This gives the problem that users often miss out on the /dev/serial/by-id part, and in addition often users don't remove the <>. This curently happens on a daily basis, if not multiple times per day.

Proposed fixes:

1. Use ls /dev/serial/by-id/* which gives the full path which can simply be copy'n'pasted
2. Provide an example as comment how the path should look
3. Tell the user to copy'n'paste explicitely, including giving the hint that marking text in PuTTy automatically copies stuff to the clipboard

[Drachenkaetzchen]

Can't self-assign but willing to work on it