Add one-line explainers for each function in InvariantTest

[PaulRBerg]

I know that documenting invariants in the Foundry Book is currently a WIP (and Lucas is handling this in foundry-rs/book#760), but I think that it would be helpful to have a few explainers in Forge Std, too.

Specifically, it would be helpful to have a few one-liners above each function in InvariantTest.

I've recently bumped into foundry-rs/foundry#2963, and apparently the fix is to use targetSender, though it's not super obvious why that fixes the sender bug. Does targetSender act as an allowlist - will Foundry fuzz only the targeted senders? And, when targetSender is not used, all senders are fuzzed by default?

Separately, how does targetSender play with excludeSender?