These two documents do a good job of explaining what is being represented on the Application Guardrails landing page and even references the internal rules that are being used to generate the logic for the warnings to be displayed.
It is also possible to view the complete inventory of warnings by looking at the instances of the Rule-Message class. As of Pega 7.1.7, Rule-Message instances encapsulate all warning messages, and you can view the entire inventory by expanding the Designer Studio Record Explorer to
Technical > Messages.
The landing page includes several filtering options to assist in finding the messages you are looking for. If you are on an earlier version than 7.1.7, the entire inventory is considerably more challenging to obtain because the warnings are hard-coded into activity-checking logic steps that fire when rules are saved.