Rollup merge of #124870 - Lokathor:update-result-docs, r=dtolnay

Update Result docs to the new guarantees

The `Option` docs already explain the guarantees given in [RFC 3391](https://github.com/rust-lang/rfcs/blob/master/text/3391-result_ffi_guarantees.md), so all that we need is a paragraph saying that some `Result` type combinations will also qualify.

Part of #110503

library/core/src/option.rs

@@ -137,10 +137,13 @@
 //!
 //! [^extern_fn]: this remains true for any argument/return types and any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`)
 //!
+//! Under some conditions the above types `T` are also null pointer optimized when wrapped in a [`Result`][result_repr].
+//!
 //! [`Box<U>`]: ../../std/boxed/struct.Box.html
 //! [`num::NonZero*`]: crate::num
 //! [`ptr::NonNull<U>`]: crate::ptr::NonNull
 //! [function call ABI]: ../primitive.fn.html#abi-compatibility
+//! [result_repr]: crate::result#representation
 //!
 //! This is called the "null pointer optimization" or NPO.
 //!

library/core/src/result.rs

@@ -228,6 +228,27 @@
 //! [`Err(E)`]: Err
 //! [io::Error]: ../../std/io/struct.Error.html "io::Error"
 //!
+//! # Representation
+//!
+//! In some cases, [`Result<T, E>`] will gain the same size, alignment, and ABI
+//! guarantees as [`Option<U>`] has. One of either the `T` or `E` type must be a
+//! type that qualifies for the `Option` [representation guarantees][opt-rep],
+//! and the *other* type must meet all of the following conditions:
+//! * Is a zero-sized type with alignment 1 (a "1-ZST").
+//! * Has no fields.
+//! * Does not have the `#[non_exhaustive]` attribute.
+//!
+//! For example, `NonZeroI32` qualifies for the `Option` representation
+//! guarantees, and `()` is a zero-sized type with alignment 1, no fields, and
+//! it isn't `non_exhaustive`. This means that both `Result<NonZeroI32, ()>` and
+//! `Result<(), NonZeroI32>` have the same size, alignment, and ABI guarantees
+//! as `Option<NonZeroI32>`. The only difference is the implied semantics:
+//! * `Option<NonZeroI32>` is "a non-zero i32 might be present"
+//! * `Result<NonZeroI32, ()>` is "a non-zero i32 success result, if any"
+//! * `Result<(), NonZeroI32>` is "a non-zero i32 error result, if any"
+//!
+//! [opt-rep]: ../option/index.html#representation "Option Representation"
+//!
 //! # Method overview
 //!
 //! In addition to working with pattern matching, [`Result`] provides a