Rollup merge of #125154 - FractalFir:fnabi_doc, r=compiler-errors

Small improvements to the documentaion of FnAbi
I have updated the documentation of `FnAbi`.
The `arg` and `ret` fields are no longer LLVM types, but Rust types(`ArgAbi` contains a `TyAndLayout` and a `PassMode`), so I changed the documentation to reflect that.
Besides that, I also added documentation to other fields, and added a clarification about the differences between `FnAbi` and `FnSig`, since this is not something that is immediately obvious.

Author: fmease

compiler/rustc_target/src/abi/call/mod.rs

@@ -779,26 +779,31 @@ impl RiscvInterruptKind {
 /// Metadata describing how the arguments to a native function
 /// should be passed in order to respect the native ABI.
 ///
+/// The signature represented by this type may not match the MIR function signature.
+/// Certain attributes, like `#[track_caller]` can introduce additional arguments, which are present in [`FnAbi`], but not in `FnSig`.
+/// While this difference is rarely relevant, it should still be kept in mind.
+///
 /// I will do my best to describe this structure, but these
 /// comments are reverse-engineered and may be inaccurate. -NDM
 #[derive(Clone,PartialEq,Eq,Hash,HashStable_Generic)]
 pubstructFnAbi<'a,Ty>{
-/// The LLVM types of each argument.
+/// The type, layout, and information about how each argument is passed.
 pubargs:Box<[ArgAbi<'a,Ty>]>,
 
-/// LLVM return type.
+/// The layout, type, and the way a value is returned from this function.
 pubret:ArgAbi<'a,Ty>,
 
+/// Marks this function as variadic (accepting a variable number of arguments).
 pubc_variadic:bool,
 
 /// The count of non-variadic arguments.
 ///
 /// Should only be different from args.len() when c_variadic is true.
 /// This can be used to know whether an argument is variadic or not.
 pubfixed_count:u32,
-
+/// The calling convention of this function.
 pubconv:Conv,
-
+/// Indicates if an unwind may happen across a call to this function.
 pubcan_unwind:bool,
 }