Rollup merge of #137322 - alexcrichton:update-wasm-docs, r=jieyouxu

Update docs for default features of wasm targets

LLVM 20 enabled the `nontrapping-fptoint` and `bulk-memory` features by default, so this updates the corresponding documentation for the `wasm32-*` targets (which all point to `wasm32-unknown-unknown`).

Closes #137315 with a doc update for the doc part.

Author: workingjubilee