Rollup merge of #149238 - RalfJung:clamp-signed-zeros, r=Amanieu

float::clamp: make treatment of signed zeros unspecified

Fixes rust-lang/rust#83984 by explicitly documenting that we do not specify the treatment of signed zeros in `clamp`. `@rust-lang/libs-api` Is this what you'd like to see?

Cc `@tgross35` `@thomcc`

Author: Zalathar