strftime.php: document %n for new lines

[Brotcrunsher]

See: https://cinnamon-spices.linuxmint.com/strftime.php

The page currently does not mention that %n can be used to insert a new line in custom date/time formats.

I only discovered this by trial and error. A small entry in the "Misc" section would have been helpful, for example:

%n - New line

Thanks!