Rollup merge of rust-lang#40907 - donniebishop:utf8_unchecked_docs, r…

…=steveklabnik

Linked str in from_utf_unchecked

References rust-lang#29375. Linked `str` in from_utf_unchecked's documentation to the docs for primitive `str`

src/libcore/str/mod.rs

@@ -327,7 +327,9 @@ unsafe fn from_raw_parts_mut<'a>(p: *mut u8, len: usize) -> &'a mut str {
 ///
 /// This function is unsafe because it does not check that the bytes passed to
 /// it are valid UTF-8. If this constraint is violated, undefined behavior
-/// results, as the rest of Rust assumes that `&str`s are valid UTF-8.
+/// results, as the rest of Rust assumes that [`&str`]s are valid UTF-8.
+///
+/// [`&str`]: ../../std/primitive.str.html
 ///
 /// # Examples
 ///