bug#37583: Documentation needed: pam-services in operating-system structure

[Jesse Gibbons]

The manual has no documentation about how to use pam-services in the
operating-system configuration. Information about how to use the pam-service 
data structure and an example of how to use it would be helpful to
administrators who need to use non-default settings, and will help us squash
the bugs we find in the future related to how guix implements pam.