Rollup merge of #99335 - Dav1dde:fromstr-docs, r=JohnTitor

Use split_once in FromStr docs

Current implementation:

```rust
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let coords: Vec<&str> = s.trim_matches(|p| p == '(' || p == ')' )
                                 .split(',')
                                 .collect();

        let x_fromstr = coords[0].parse::<i32>()?;
        let y_fromstr = coords[1].parse::<i32>()?;

        Ok(Point { x: x_fromstr, y: y_fromstr })
    }
```

Creating the vector is not necessary, `split_once` does the job better.

Alternatively we could also remove `trim_matches` with `strip_prefix` and `strip_suffix`:

```rust
        let (x, y) = s
            .strip_prefix('(')
            .and_then(|s| s.strip_suffix(')'))
            .and_then(|s| s.split_once(','))
            .unwrap();
```

The question is how much 'correctness' is too much and distracts from the example. In a real implementation you would also not unwrap (or originally access the vector without bounds checks), but implementing a custom Error and adding a `From<ParseIntError>` and implementing the `Error` trait adds a lot of code to the example which is not relevant to the `FromStr` trait.

core/src/str/traits.rs

@@ -519,12 +519,14 @@ unsafe impl const SliceIndex<str> for ops::RangeToInclusive<usize> {
 ///     type Err = ParseIntError;
 ///
 ///     fn from_str(s: &str) -> Result<Self, Self::Err> {
-///         let coords: Vec<&str> = s.trim_matches(|p| p == '(' || p == ')' )
-///                                  .split(',')
-///                                  .collect();
-///
-///         let x_fromstr = coords[0].parse::<i32>()?;
-///         let y_fromstr = coords[1].parse::<i32>()?;
+///         let (x, y) = s
+///             .strip_prefix('(')
+///             .and_then(|s| s.strip_suffix(')'))
+///             .and_then(|s| s.split_once(','))
+///             .unwrap();
+///
+///         let x_fromstr = x.parse::<i32>()?;
+///         let y_fromstr = y.parse::<i32>()?;
 ///
 ///         Ok(Point { x: x_fromstr, y: y_fromstr })
 ///     }