Rollup merge of #104436 - ismailmaj:add-slice-to-stack-allocated-string-comment, r=Mark-Simulacrum

Add slice to the stack allocated string comment

Precise that the "stack allocated string" is not a string but a string slice.

``@rustbot`` label +A-docs

library/core/src/str/converts.rs

@@ -77,7 +77,7 @@ use super::Utf8Error;
 /// let sparkle_heart = [240, 159, 146, 150];
 ///
 /// // We know these bytes are valid, so just use `unwrap()`.
-/// let sparkle_heart = str::from_utf8(&sparkle_heart).unwrap();
+/// let sparkle_heart: &str = str::from_utf8(&sparkle_heart).unwrap();
 ///
 /// assert_eq!("💖", sparkle_heart);
 /// ```