LLVM CFI integration being undocumented complicates further codegen dev

[workingjubilee]

When #105452 and related PRs added a bunch of parameters to things like BuilderMethods::call, it did not include explanations of what those parameters were for. Namely, that they were for CFI tracking and that they were not relevant to actual ABI handling.

The knowledge exists in the heads of Rust contributors, of course, so we can just talk to each other... except for how it is in the heads of Rust contributors who are very busy because they have a lot of different things they have to attend to because they're the only one who knows the thing.

I would like to see these just-for-CFI changes documented and ideally cleaned up so we are no longer passing ambiguous details around which are secretly-only-advisory-except-if-you-are-doing-CFI-related-codegen-in-which-case-it-is-critical.

cc @rcvalle

[workingjubilee]

Opening this as an issue because I am trying to partially address this in #143716 but I know there are like 4+ other PRs and most of them made somewhat sweeping changes.