Clean up E0121 long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0121.md

@@ -1,10 +1,24 @@
-In order to be consistent with Rust's lack of global type inference,
-type and const placeholders are disallowed by design in item signatures.
+The type placeholder `_` was used withing a type on an item's signature.
 
-Examples of this error include:
+Erroneous code example:
 
 ```compile_fail,E0121
-fn foo() -> _ { 5 } // error, explicitly write out the return type instead
+fn foo() -> _ { 5 } // error
 
-static BAR: _ = "test"; // error, explicitly write out the type instead
+static BAR: _ = "test"; // error
+```
+
+In those cases, you need to provide the type explicitly:
+
+```
+fn foo() -> i32 { 5 } // ok!
+
+static BAR: &str = "test"; // ok!
+```
+
+The type placeholder `_` can be used outside item's signature as follows:
+
+```
+let x = "a4a".split('4')
+    .collect::<Vec<_>>(); // No need to precise the Vec's generic type.
 ```