Update `#[inline]` guidance

[ChrisDenton]

Our current guidance on using #[inline] should mention profiling as our guiding principle even in places where we currently think the attribute "shouldn't" be needed (and maybe we want to revisit those "shouldn't"s). I think it can be inferred from the docs that making changes to #[inline] can be a separate PR to other changes but we should make that more explicit.

See also rust-lang/rust#109546. cc @saethlin