Rollup merge of rust-lang#74486 - poliorcetics:read-exact-doc-point-to-read, r=Mark-Simulacrum

Improve Read::read_exact documentation

Fixes rust-lang#72186.

For now I simply added a link to `Read::read` and held off changing the text in `Read::read_exact`. I can do it if it is deemed necessary.

@rustbot modify labels: C-enhancement, T-libs

Author: Manishearth

src/libstd/io/mod.rs

@@ -722,7 +722,9 @@ pub trait Read {
     /// No guarantees are provided about the contents of `buf` when this
     /// function is called, 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.
+    /// only write data to `buf` instead of reading its contents. The
+    /// documentation on [`read`] has a more detailed explanation on this
+    /// subject.
     ///
     /// # Errors
     ///
@@ -745,6 +747,7 @@ pub trait Read {
     ///
     /// [`File`]s implement `Read`:
     ///
+    /// [`read`]: Read::read
     /// [`File`]: crate::fs::File
     ///
     /// ```no_run