Improve documentation for str::replace() and str::replacen()

Currently, to know what the function will return when the pattern
doesn't match, the docs require the reader to understand the
implementation detail and mentally evaluate or run the example
code. It is not immediately clear.

This PR makes it more explicit so the reader can quickly find the
information.

library/alloc/src/str.rs

@@ -256,7 +256,7 @@ impl str {
     /// assert_eq!("than an old", s.replace("is", "an"));
     /// ```
     ///
-    /// When the pattern doesn't match:
+    /// When the pattern doesn't match, it returns this string slice as [`String`]:
     ///
     /// ```
     /// let s = "this is old";
@@ -297,7 +297,7 @@ impl str {
     /// assert_eq!("foo foo new23 foo", s.replacen(char::is_numeric, "new", 1));
     /// ```
     ///
-    /// When the pattern doesn't match:
+    /// When the pattern doesn't match, it returns this string slice as [`String`]:
     ///
     /// ```
     /// let s = "this is old";