Correct safety reasoning in str::make_ascii_{lower,upper}case()

Author: ChayimFriedman2

library/core/src/str/mod.rs

@@ -2407,7 +2407,7 @@ impl str {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn make_ascii_uppercase(&mut self) {
-        // SAFETY: safe because we transmute two types with the same layout.
+        // SAFETY: changing ASCII letters only does not invalidate UTF-8.
         let me = unsafe { self.as_bytes_mut() };
         me.make_ascii_uppercase()
     }
@@ -2434,7 +2434,7 @@ impl str {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn make_ascii_lowercase(&mut self) {
-        // SAFETY: safe because we transmute two types with the same layout.
+        // SAFETY: changing ASCII letters only does not invalidate UTF-8.
         let me = unsafe { self.as_bytes_mut() };
         me.make_ascii_lowercase()
     }