Rollup merge of rust-lang#141370 - WaffleLapkin:hiiii, r=jhpratt

add doc alias `replace_first` for `str::replacen`

`replace_first` is a sensible name for a function, analogous to actually existing `<[_]>::split_first`, for example. (I just saw someone try to search for it)

I think it's reasonable to add such an alias for `replacen`, which replaces the first occurrence of passed a 1.

Author: matthiaskrgr

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")]