add doc alias replace_first for str::replacen

Author: WaffleLapkin

library/alloc/src/str.rs

@@ -320,6 +320,7 @@ impl str {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[rustc_allow_incoherent_impl]
+    #[doc(alias = "replace_first")]
     #[must_use = "this returns the replaced string as a new allocation, \
                   without modifying the original"]
     #[stable(feature = "str_replacen", since = "1.16.0")]