docs: Enhance polkit documentation to describe secondary connection

docs: Enhance polkit documentation to describe secondary connection

https://bugzilla.redhat.com/show_bug.cgi?id=1631606

Since commit 8259255 usage of a primary connection driver for
a virConnect has been modified to open (virConnectOpen) and use
a connection to the specific driver in order to handle the API
calls to/for that driver. This causes some confusion and issues
for ACL polkit rule scripts to know exactly which driver by
name will be used.

Add some documentation describing the processing of the primary
and secondary connection as well as the list of the connect_driver
names used for each driver.

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>