Add a link to read in the read_exact doc about the guarantees

src/libstd/io/mod.rs

@@ -741,7 +741,9 @@ fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
     /// 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
     ///
@@ -764,6 +766,7 @@ fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
     ///
     /// [`File`]s implement `Read`:
     ///
+    /// [`read`]: trait.Read.html#tymethod.read
     /// [`File`]: ../fs/struct.File.html
     /// [`ErrorKind::Interrupted`]: ../../std/io/enum.ErrorKind.html#variant.Interrupted
     /// [`ErrorKind::UnexpectedEof`]: ../../std/io/enum.ErrorKind.html#variant.UnexpectedEof