Elaborate on StrRead::parse_str safety comment

src/read.rs

@@ -654,8 +654,9 @@ impl<'a> Read<'a> for StrRead<'a> {
 
     fn parse_str<'s>(&'s mut self, scratch: &'s mut Vec<u8>) -> Result<Reference<'a, 's, str>> {
         self.delegate.parse_str_bytes(scratch, true, |_, bytes| {
-            // The input is assumed to be valid UTF-8 and the \u-escapes are
-            // checked along the way, so don't need to check here.
+            // The deserialization input came in as &str with a UTF-8 guarantee,
+            // and the \u-escapes are checked along the way, so don't need to
+            // check here.
             Ok(unsafe { str::from_utf8_unchecked(bytes) })
         })
     }