Clarify Source.amplify

[expenses]

When reading over the documentation, I naively assumed that Source.amplify would amplify a source by a value in decibels, similar to how amplifying a track in Audacity works, but instead it sets the volume for the source the same way that Sink.set_volume does (additionally, passing a negative values functions the same as passing a positive values, which didn't help).

I think it would be useful to clarify this so that people don't make the same mistake!