Extra documentation of new formatting feature

[IsaacCloos]

Documentation of this feature was added in #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
Feature PR - 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 😄

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