Rollup merge of #111940 - zirconium-n:io-read-doc-change, r=thomcc

Clarify safety concern of `io::Read::read` is only relevant in unsafe code

We have this clarification note in other similar place like [Iterator::size_hint](https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.size_hint).

The lack of clarification might lead to confusion to Rust beginners. [Relevant URLO post](https://users.rust-lang.org/t/can-read-overflow-a-buffer/94347).

library/std/src/io/mod.rs

@@ -593,7 +593,8 @@ pub trait Read {
     /// This may happen for example because fewer bytes are actually available right now
     /// (e. g. being close to end-of-file) or because read() was interrupted by a signal.
     ///
-    /// As this trait is safe to implement, callers cannot rely on `n <= buf.len()` for safety.
+    /// As this trait is safe to implement, callers in unsafe code cannot rely on
+    /// `n <= buf.len()` for safety.
     /// Extra care needs to be taken when `unsafe` functions are used to access the read bytes.
     /// Callers have to ensure that no unchecked out-of-bounds accesses are possible even if
     /// `n > buf.len()`.
@@ -603,8 +604,8 @@ pub trait Read {
     /// contents of `buf` being true. It is recommended that *implementations*
     /// only write data to `buf` instead of reading its contents.
     ///
-    /// Correspondingly, however, *callers* of this method must not assume any guarantees
-    /// about how the implementation uses `buf`. The trait is safe to implement,
+    /// Correspondingly, however, *callers* of this method in unsafe code must not assume
+    /// any guarantees about how the implementation uses `buf`. The trait is safe to implement,
     /// so it is possible that the code that's supposed to write to the buffer might also read
     /// from it. It is your responsibility to make sure that `buf` is initialized
     /// before calling `read`. Calling `read` with an uninitialized `buf` (of the kind one