geek/schnapps: fix schnapps encryption usage guide
Reflect the actual way Schnapps uses GPG to encrypt snapshot exports. The archive is just symmetrically encrypted with the chosen password, not by a GPG key encrypted with the password.
See https://gitlab.nic.cz/turris/schnapps/-/blob/v2.8.0/schnapps.sh#L571-580
Therefore, among other things, there is no GPG setup required except for "gnupg" (GPG 1.4) package installation (should be added as a dependency to Schnapps, but currently is not - turris/os/packages#841 (closed) ).
Additionally a smaller fix:
geek/schnapps: fix keep.max_* config descriptions
replace "single" type everywhere with actually referred to types