stdlib docs: document lifetime extension for `format_args!`'s arguments
Since rust-lang/rust#140748 is stable and rust-lang/rust#92698 is closed, the section about `format_args!`'s argument lifetime limitation is outdated. I've updated it to point to the Reference docs, which will specify lifetime extension rules for builtin macros once rust-lang/reference#1980 or equivalent is merged.
I've also taken the liberty of updating one of the doctests to assign the result of `format_args!` to a variable, both to provide an example and because I think it reads a little better.
r? `@m-ou-se`
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