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.

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)]
 pub struct FnAbi<'a, Ty> {
-    /// The LLVM types of each argument.
+    /// The type, layout, and information about how each argument is passed.
     pub args: Box<[ArgAbi<'a, Ty>]>,
 
-    /// LLVM return type.
+    /// The layout, type, and the way a value is returned from this function.
     pub ret: ArgAbi<'a, Ty>,
 
+    /// Marks this function as variadic (accepting a variable number of arguments).
     pub c_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.
     pub fixed_count: u32,
-
+    /// The calling convention of this function.
     pub conv: Conv,
-
+    /// Indicates if an unwind may happen across a call to this function.
     pub can_unwind: bool,
 }