Modify what priority is assigned to downloads

[chros73]

So far, special group got high priority and the rest of the downloads got normal priority.

Change it in rtorrent config to:

• special group gets normal priority
• rest of the downloads gets low priority

Also change the corresponding wiki page about this.

The reason behind this change is that in this way we can easily set a favorite download to high priority with + key, so it can be downloaded faster while all other downloads are still downloading.

Migration instructions:

• use the following commands to modify all the existing downloads to the new logic and save their session

d.multicall2=main,"branch=d.throttle_name=,d.priority.set=1"
d.multicall2=main,"branch=((not,(d.throttle_name))),d.priority.set=2"
session.save=