Rollup merge of #106823 - m-ou-se:format-args-as-str-guarantees, r=dtolnay

Allow fmt::Arguments::as_str() to return more Some(_).

This adjusts the documentation to allow optimization of format_args!() to be visible through fmt::Arguments::as_str().

This allows for future changes like https://github.com/rust-lang/rust/pull/106824.

diff --cc library/core/src/fmt/mod.rs
Simple merge