Merge pull request #4074 from spotlesscoder/patch-1

fix: make the reason more understandable

Author: chriskrycho

src/ch04-03-slices.md

@@ -16,10 +16,12 @@ slices, to understand the problem that slices will solve:
 fn first_word(s: &String) -> ?
 ```
 
-The `first_word` function has a `&String` as a parameter. We don’t want
-ownership, so this is fine. But what should we return? We don’t really have a
-way to talk about _part_ of a string. However, we could return the index of the
-end of the word, indicated by a space. Let’s try that, as shown in Listing 4-7.
+The `first_word` function has a `&String` as a parameter. We don’t need
+ownership, so this is fine. (In idiomatic Rust, functions do not take ownership
+of their arguments unless they need to, and the reasons for that will become
+clear as we keep going!) But what should we return? We don’t really have a way
+to talk about part of a string. However, we could return the index of the end of
+the word, indicated by a space. Let’s try that, as shown in Listing 4-7.
 
 <Listing number="4-7" file-name="src/main.rs" caption="The `first_word` function that returns a byte index value into the `String` parameter">