Rollup merge of #82892 - jix:clarify-read-read, r=joshtriplett

Rollup merge of #82892 - jix:clarify-read-read, r=joshtriplett

Clarify docs for Read::read's return value

Right now the docs for `Read::read`'s return value are phrased in a way that makes it easy for the reader to assume that the return value is never larger than the passed buffer. This PR clarifies that this is a requirement for implementations of the trait, but that callers have to expect a buggy yet safe implementation failing to do so, especially if unchecked accesses to the buffer are done afterwards.

I fell into this trap recently, and when I noticed, I looked at the docs again and had the feeling that I might not have been the first one to miss this.

The same issue of trusting the return value of `read` was also present in std itself for about 2.5 years and only fixed recently, see #80895.

I hope that clarifying the docs might help others to avoid this issue.