Add note about safety in documentation

Co-Authored-By: Michael Schmidt <[email protected]>

Author: daxpedda

Diff for: guide/src/reference/types/boxed-number-slices.md

@@ -8,6 +8,10 @@
 from the Wasm linear memory when returning a boxed slice to JavaScript, and vice
 versa when receiving a JavaScript `TypedArray` as a boxed slice in Rust.
 
+> **Note:** Numeric `MaybeUninit<T>` can always be assumed to be initialized
+> upon transmission from Rust to JS and vice-versa. However, uninitialized
+> values coming from Rust might contain unspecified values.
+
 ## Example Rust Usage
 
 ```rust

Diff for: guide/src/reference/types/number-slices.md

@@ -4,6 +4,10 @@
 |:---:|:---:|:---:|:---:|:---:|:---:|:---:|
 | No | Yes | Yes | No | No | No | A JavaScript `TypedArray` view of the Wasm memory for the boxed slice of the appropriate type (`Int32Array`, `Uint8Array`, etc) |
 
+> **Note:** Numeric `MaybeUninit<T>` can always be assumed to be initialized
+> upon transmission from Rust to JS and vice-versa. However, uninitialized
+> values coming from Rust might contain unspecified values.
+
 ## Example Rust Usage
 
 ```rust