Rollup merge of #87685 - notriddle:lazy-from-docs, r=dtolnay
Rollup merge of #87685 - notriddle:lazy-from-docs, r=dtolnay

Write docs for SyncOnceCell From and Default impl

Part of #51430

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