Merge pull request #2357 from rust-lang/gh1785

Clarify some wording a bit

Author: steveklabnik

src/ch08-01-vectors.md

@@ -138,7 +138,7 @@ and any other references to the contents of the vector remain valid. Recall the
 rule that states you can’t have mutable and immutable references in the same
 scope. That rule applies in Listing 8-7, where we hold an immutable reference to
 the first element in a vector and try to add an element to the end, which won’t
-work.
+work if we also try to refer to that element later in the function:
 
 ```rust,ignore,does_not_compile
 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-07/src/main.rs:here}}