Global RBAC permissions needs to be documented

[minsis]

Carrying part of the conversion over from StackStorm/st2#5303 (comment)

The global RBAC permission types are not documented. While there is some overlap for types within some of the resources there should be a section specifying the global settings as well.

[arm4b]

Thanks for reporting this!

[amanda11]

In st2docs we generate the available_permission_types.rst by going through each resource and finding the permission types for each resource. But we don't go through the st2common.rbac.types.GLOBAL_PERMISSION_TYPES and generate that list.
Suggest amending generate-available-permission-types-table.py to generate the global list before it adds the resource lists.
I can try and look at that if that sounds like a plan...