Rollup merge of #85302 - r00ster91:patch-7, r=joshtriplett

Expand WASI abbreviation in docs

I was pretty sure this was related to something for WebAssembly but wasn't 100% sure so I checked but even on these top-level docs I couldn't find the abbreviation expanded. I'm normally used to Rust docs being detailed and explanatory and writing abbreviations like this out in full at least once so I thought it was worth the change. Feel free to close this if it's too much.