Rollup merge of rust-lang#100855 - IsaacCloos:master, r=joshtriplett

Extra documentation for new formatting feature

Documentation of this feature was added in rust-lang#90473 and released in Rust 1.58. However, high traffic macros did not receive new examples. Namely `println!()` and `format!()`.

The doc comments included in Rust are super important to the community- especially newcomers. I have met several other newbies like myself who are unaware of this recent (well about 7 months old now) update to the language allowing for convenient intra-string identifiers.

Bringing small examples of this feature to the doc comments of `println!()` and `format!()` would be helpful to everyone learning the language.

[Blog Post Announcing Feature](https://blog.rust-lang.org/2022/01/13/Rust-1.58.0.html)
[Feature PR](rust-lang#90473) - includes several instances of documentation of the feature- minus the macros in question for this PR

*This is my first time contributing to a project this large. Feedback would mean the world to me 😄*

---

*Recreated; I violated the [No-Merge Policy](https://rustc-dev-guide.rust-lang.org/git.html#no-merge-policy)*

Author: matthiaskrgr

library/alloc/src/macros.rs

@@ -107,6 +107,8 @@ macro_rules! vec {
 /// format!("test");
 /// format!("hello {}", "world!");
 /// format!("x = {}, y = {y}", 10, y = 30);
+/// let (x, y) = (1, 2);
+/// format!("{x} + {y} = 3");
 /// ```
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]

library/std/src/macros.rs

@@ -93,6 +93,8 @@ macro_rules! print {
 /// println!(); // prints just a newline
 /// println!("hello there!");
 /// println!("format {} arguments", "some");
+/// let local_variable = "some";
+/// println!("format {local_variable} arguments");
 /// ```
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]