Rollup merge of #79860 - rust-lang:frewsxcv-patch-2, r=jyn514

Clarify that String::split_at takes a byte index.

To someone skimming through the `String` docs and only reads the first line, the person could interpret "index" to be "char index". Later on in the docs it clarifies, but by adding "byte" it removes that ambiguity.

Author: tmandry

Diff for: library/alloc/src/string.rs

@@ -1413,7 +1413,7 @@ impl String {
         self.len() == 0
     }
 
-    /// Splits the string into two at the given index.
+    /// Splits the string into two at the given byte index.
     ///
     /// Returns a newly allocated `String`. `self` contains bytes `[0, at)`, and
     /// the returned `String` contains bytes `[at, len)`. `at` must be on the