Merge pull request rust-lang#752 from rust-lang/leading-modsep-2018

clarify note re. leading `::` in 2018

src/paths.md

@@ -175,7 +175,8 @@ mod a {
 }
 mod b {
     pub fn foo() {
-        ::a::foo(); // call a's foo function
+        ::a::foo(); // call `a`'s foo function
+        // In Rust 2018, `::a` would be interpreted as the crate `a`.
     }
 }
 # fn main() {}