doc: add priority equation to accounting guide

[garlick]

Problem: the accounting guide uses words to describe how priority is calculated but math would be clearer.

Add "queue" to the lists of factors and format the list as a dictionary rather than a bullet list.

Use a math expression for the priority value.

I briefly looked at the code and priority appeared to be calculated this way - did I get it right?

[garlick]

(oops fixed a typo and forced a push)

[garlick]

Just to confirm, so the urgency factor has a weight of 1 and we subtract 16 from it before adding?

[cmoussa1]

You are correct, it has a weight of 1.

[garlick]

OK, I'll set MWP then. Thanks!