Document rust_2015 methods

Author: WaffleLapkin

compiler/rustc_session/src/session.rs

@@ -899,6 +899,7 @@ impl Session {
         ret
     }
 
+    /// Is this edition 2015?
     pub fn rust_2015(&self) -> bool {
         self.edition().rust_2015()
     }

compiler/rustc_span/src/edition.rs

@@ -76,6 +76,7 @@ impl Edition {
         }
     }
 
+    /// Is this edition 2015?
     pub fn rust_2015(&self) -> bool {
         *self == Edition::Edition2015
     }