accounting: write docs about configuring priority factor weights in TOML file

[cmoussa1]

Once #295 is merged, I should probably create some documentation in the flux-accounting section outlining how to configure the integer weights for the numerous priority factors in the multi-factor priority plugin, maybe along with an example. In that PR, priority factor weights can be configured in a TOML file like so:

[priority_factors]
fshare_weight = 1000
queue_weight = 100
age_weight = 10