manpage: add example for crypt_root and crypt_swap options

[yamabiiko]

I am opening this PR to add an example in the commit mentioned options to clarify possible confusion for future users, as it wasn't clear that I had to specify "--option,--option2", instead of "option1,option2" (for example, in mkinitcpio they use the latter system, specifically cryptdevice=device:dmname:option1,option2)