Document that untagged enums equire std or alloc features

I was happily writing some code that needs to support no_std and no_alloc only to find out that it's not possible through the compilation simply failing:

```rust
error[E0433]: failed to resolve: could not find `ContentRefDeserializer` in `de`
   --> zarlink/src/connection/mod.rs:136:36
    |
136 |         #[derive(Debug, Serialize, Deserialize)]
    |                                    ^^^^^^^^^^^ could not find `ContentRefDeserializer` in `de`
    |
note: found an item that was configured out
   --> /home/zeenix/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.218/src/private/de.rs:14:35
    |
14  |     Content, ContentDeserializer, ContentRefDeserializer, EnumDeserializer,
    |                                   ^^^^^^^^^^^^^^^^^^^^^^
note: the item is gated here
   --> /home/zeenix/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.218/src/private/de.rs:12:1
    |
12  | #[cfg(any(feature = "std", feature = "alloc"))]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0412]: cannot find type `Content` in module `_serde::__private::de`
   --> zarlink/src/connection/mod.rs:136:36
    |
136 |         #[derive(Debug, Serialize, Deserialize)]
    |                                    ^^^^^^^^^^^ not found in `_serde::__private::de`
    |
note: found an item that was configured out
   --> /home/zeenix/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.218/src/private/de.rs:14:5
    |
14  |     Content, ContentDeserializer, ContentRefDeserializer, EnumDeserializer,
    |     ^^^^^^^
note: the item is gated here
   --> /home/zeenix/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.218/src/private/de.rs:12:1
    |
12  | #[cfg(any(feature = "std", feature = "alloc"))]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)
```

Author: zeenix

_src/enum-representations.md

@@ -145,3 +145,5 @@ enum Data {
 #
 # fn main() {}
 ```
+
+**Note**: Untagged enums require either `std` or `alloc` Cargo features to be enabled.