Rollup merge of #70776 - RalfJung:raw-vec, r=Dylan-DPC,TimDiekmann
Rollup merge of #70776 - RalfJung:raw-vec, r=Dylan-DPC,TimDiekmann

clarify comment in RawVec::into_box

On first reading I almost thought "len <= cap" would be all that there is to check here. Expand the comment to clarify that that is not the case.

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