Update Error 0432 long description for Rust 2018

src/librustc_error_codes/error_codes/E0432.md

@@ -6,9 +6,13 @@ Erroneous code example:
 use something::Foo; // error: unresolved import `something::Foo`.
 ```
 
-Paths in `use` statements are relative to the crate root. To import items
+In Rust 2015, paths in `use` statements are relative to the crate root. To import items
 relative to the current and parent modules, use the `self::` and `super::`
-prefixes, respectively. Also verify that you didn't misspell the import
+prefixes, respectively.
+
+In Rust 2018, paths in `use` statements are relative to the current module unless they begin with the name of a crate or a literal `crate::`, in which case they start from the crate root. As in Rust 2015 code, the `self::` and `super::` prefixes refer to the current and parent modules.
+
+Also verify that you didn't misspell the import
 name and that the import exists in the module from where you tried to
 import it. Example:
 
@@ -21,12 +25,19 @@ mod something {
 # fn main() {}
 ```
 
-Or, if you tried to use a module from an external crate, you may have missed
+Or, if you tried to use a module from an external crate and are using Rust 2015, you may have missed
 the `extern crate` declaration (which is usually placed in the crate root):
 
 ```
-extern crate core; // Required to use the `core` crate
+extern crate core; // Required to use the `core` crate in Rust 2015.
 
 use core::any;
 # fn main() {}
 ```
+
+In Rust 2018, the `extern crate` declaration is not required and you can instead just `use` it:
+
+```
+use core::any; // No extern crate required in Rust 2018!
+# fn main() {}
+```