Rollup merge of rust-lang#40992 - donniebishop:utf8err_linking, r=ale…

…xcrichton

Added links to from_utf8 methods in Utf8Error

Referencing  rust-lang#29375. Linked the `from_utf8` methods for both `String` and `str` in the description. Also linked the `u8` to its documentation

src/libcore/str/mod.rs

@@ -152,11 +152,16 @@ impl fmt::Display for ParseBoolError {
 Section: Creating a string
 */
 
-/// Errors which can occur when attempting to interpret a sequence of `u8`
+/// Errors which can occur when attempting to interpret a sequence of [`u8`]
 /// as a string.
 ///
-/// As such, the `from_utf8` family of functions and methods for both `String`s
-/// and `&str`s make use of this error, for example.
+/// [`u8`]: ../../std/primitive.u8.html
+///
+/// As such, the `from_utf8` family of functions and methods for both [`String`]s
+/// and [`&str`]s make use of this error, for example.
+///
+/// [`String`]: ../../std/string/struct.String.html#method.from_utf8
+/// [`&str`]: ../../std/str/fn.from_utf8.html
 #[derive(Copy, Eq, PartialEq, Clone, Debug)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Utf8Error {