Rollup merge of #82837 - RalfJung:maybe-uninit, r=dtolnay
Rollup merge of #82837 - RalfJung:maybe-uninit, r=dtolnay

tweak MaybeUninit docs

Explain what "(no) fixed value" means.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust