Explain what the `fn` is an "ABI" and what equivalencies we guarantee

[workingjubilee]

Documentation for this is sort of awkwardly scattered around std docs, the Reference, the Rustonomicon, and the UCG (some of which is more aspirational than actual, for note!), we should sit down and actually document it all somewhere. Probably a new or massively-expanded section of the Reference.

Some notes on major things to make sure we've gotten down into roughly the same place:

• Explain the difference between repr(Rust) and extern "Rust" or repr(C) and extern "C"
• Explain how Rust ABI strings relate to commonly-described ABI (e.g. "C" vs. "C-unwind")
• Explain that memory layout is important to, but not sufficient for, matching function-call ABI
• Explain that extern "C" has a target-specific meaning based on a psABI and how it is interpreted by the compilers, not specified by "Standard C"
• Document the special-case guarantees for some types:
  • Option<T>
  • Result<T, Zst>
  • Result<Zst, T>
  • bool
• Explain the ABI-mismatch UB cases
  • Document any major "unguarantees" (reservations?)
• Do we have to mention #[target_feature]?
• Explain that there are not-immediately-arg/ret-related esoterica about ABI
  • mention stack alignment
  • mention MXCSR or FPCSR state
  • mention unwinding / destructors

[workingjubilee]

@chorman0773 you'll fill in the list of things I missed, I trust

[chorman0773]

#1545 contains the ABI equivalence rules.

[workingjubilee]

Cool!