Add Documentation for Global Log Filters

[dutchrapley]

Describe the bug Global log filters were added in Rundeck 3.1.0 but they were never documented in the documentation.

Source page https://github.com/rundeck/rundeck/pull/4806

[fdevans]

Not sure if this is covered or not? Passing Variables doc may help?

[jgarces-pd]

Not sure if this is covered or not? Passing Variables doc may help?

it should be documented as a whole, how to configure it, which options are available, etc