Rollup merge of #70588 - Coder-256:str-split-at-docs, r=Dylan-DPC

Fix incorrect documentation for `str::{split_at, split_at_mut}`

The documentation for each method currently states:

> Panics if `mid` is not on a UTF-8 code point boundary, or if it is beyond the last code point of the string slice.

However, this is not consistent with the real behavior, or that of the corresponding methods for `[T]` slices. A comment inside each of the `str` methods states:

> is_char_boundary checks that the index is in [0, .len()]

That is what I would expect the behavior to be, and in fact this seems to be the real behavior. For example ([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=8e03dcc209d4dd176df2297523f9fee1)):

```rust
fn main() {
    // Prints ("abc", "") and doesn't panic
    println!("{:?}", "abc".split_at(3));
}
```

In this case, I would interpret "the last code point of the string slice" to mean the byte at index 2 in UTF-8. However, it is possible to pass an index of 3, which is definitely "beyond the last code point of the string slice".

I think that this is much clearer, but feel free to bikeshed.

src/libcore/str/mod.rs

@@ -2642,7 +2642,7 @@ pub unsafe fn slice_mut_unchecked(&mut self, begin: usize, end: usize) -> &mut s
     /// # Panics
     ///
     /// Panics if `mid` is not on a UTF-8 code point boundary, or if it is
-    /// beyond the last code point of the string slice.
+    /// past the end of the last code point of the string slice.
     ///
     /// # Examples
     ///
@@ -2683,7 +2683,7 @@ pub fn split_at(&self, mid: usize) -> (&str, &str) {
     /// # Panics
     ///
     /// Panics if `mid` is not on a UTF-8 code point boundary, or if it is
-    /// beyond the last code point of the string slice.
+    /// past the end of the last code point of the string slice.
     ///
     /// # Examples
     ///