Prevent <= 0 values for automatic fetching option in Preferences UI (+extend description)

[themilkman]

Problem description

Seems a bit weird that the automatic fetching input accepts negative numbers and zero. Also it is undocumented in the help text how this would be handled.
I know one could enter it in the Preferences file, but at least the UI should be more restrictive imho.

Preferred solution

The explanation text below the setting should mention what "zero" does and the UI should catch negative numbers (and maybe convert to 0 ?)