Documentation: firmware: Clarify firmware path usage

Newline characters will be taken into account for the firmware search
path parameter, warn users about that and provide an example using 'echo
-n' such that it clarifies the typical use of that parameter.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230402135423.3235-1-f.fainelli@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>