certmonger: move 'criteria' description to module docstring
The 'criteria' parameter is used by several subroutines in the
ipalib.install.certmonger module. It has incomplete documentation
spread across several of these subroutines. Move the documentation
to the module docstring and reference it where appropriate.
Part of: https://pagure.io/freeipa/issue/8186
Reviewed-By: Rob Crittenden <rcritten@redhat.com>