Unrolled build for rust-lang#122276

Rollup merge of rust-lang#122276 - RalfJung:io-read, r=Nilstrieb

io::Read trait: make it more clear when we are adressing implementations vs callers

Inspired by [this](rust-lang#72186 (comment)) comment.

For some reason we only have that `buf` warning in `read` and `read_exact`, even though it affects a bunch of other functions of this trait as well. It doesn't seem worth copy-pasting the same text everywhere though so I did not change this.

Author: rust-timer

library/std/src/io/mod.rs

@@ -692,10 +692,9 @@ pub trait Read {
     /// Callers have to ensure that no unchecked out-of-bounds accesses are possible even if
     /// `n > buf.len()`.
     ///
-    /// No guarantees are provided about the contents of `buf` when this
-    /// function is called, so implementations cannot rely on any property of the
-    /// contents of `buf` being true. It is recommended that *implementations*
-    /// only write data to `buf` instead of reading its contents.
+    /// *Implementations* of this method can make no assumptions about the contents of `buf` when
+    /// this function is called. It is recommended that implementations only write data to `buf`
+    /// instead of reading its contents.
     ///
     /// 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,
@@ -901,12 +900,10 @@ pub trait Read {
     /// This function reads as many bytes as necessary to completely fill the
     /// specified buffer `buf`.
     ///
-    /// No guarantees are provided about the contents of `buf` when this
-    /// function is called, so implementations cannot rely on any property of the
-    /// contents of `buf` being true. It is recommended that implementations
-    /// only write data to `buf` instead of reading its contents. The
-    /// documentation on [`read`] has a more detailed explanation on this
-    /// subject.
+    /// *Implementations* of this method can make no assumptions about the contents of `buf` when
+    /// this function is called. It is recommended that implementations only write data to `buf`
+    /// instead of reading its contents. The documentation on [`read`] has a more detailed
+    /// explanation of this subject.
     ///
     /// # Errors
     ///