What should I set the fuel petcock to when the bike is parked in my garage? I see 3 settings, Res (reserve), Pri (no idea what this is, prime?), and On (this one is obvious).

What does Pri stand for? And, is that the setting I should put the petcock on when the bike is parked?

ON.

You are correct, PRI is prime. You should read the wiki. Quick way to get up to speed on a lot of topics.

Since it's a vacuum petcock both On and Res are appropriate parking positions.

Pri bypasses the vacuum and is used in emergencies, diagnostics, and when first taking the bike out of storage.