Rollup merge of #76555 - alilleybrinker:reword_trivial_casts_lint_doc, r=steveklabnik
Reword `trivial_casts` lint in rustc book to better explain what it does. The current description of the trivial casts lint under the "allowed by default" listing in the rustc book indicates the lint is for casts which may be removed, which is less clear than saying it's for casts which may be replaced by coercion (which is the wording used by the error message included in the doc). This commit changes the wording slightly to better describe what the lint does. This issue bit me in some recent code where I was attempting to convert a `Vec<SomeType>` to a `Vec<SomeTraitObject>`, and hit my project-wide `#![deny(trivial_casts)]` with `map(|o| Box::new(o) as TraitObject)`. I'd read the book docs for `trivial_casts` and was surprised by the error, as I took it to mean the cast ought to be removed (rather than replaced by ascription in this case). Removing the cast meant other code didn't compile, and I then found issues like #23742 and realized my misunderstanding.
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