Rollup merge of rust-lang#40226 - jdhorwitz:master, r=frewsxcv

Issue rust-lang#39688 - Help people find String::as_bytes() for UTF-8

Added in links for the inverse functions so people will know that as_bytes() is the inverse of from_utf8() and vice versa.
?r @steveklabnik

src/libcollections/string.rs

@@ -433,6 +433,10 @@ impl String {
     ///
     /// [`str::from_utf8()`]: ../../std/str/fn.from_utf8.html
     ///
+    /// The inverse of this method is [`as_bytes`].
+    ///
+    /// [`as_bytes`]: #method.as_bytes
+    ///
     /// # Errors
     ///
     /// Returns `Err` if the slice is not UTF-8 with a description as to why the
@@ -979,6 +983,10 @@ impl String {
 
     /// Returns a byte slice of this `String`'s contents.
     ///
+    /// The inverse of this method is [`from_utf8`].
+    ///
+    /// [`from_utf8`]: #method.from_utf8
+    ///
     /// # Examples
     ///
     /// Basic usage: