Rollup merge of #44296 - zmanian:patch-1, r=steveklabnik

Minor documentation improvements for StmtKind

Documentation for Semi and Marco StmtKinds.

Wasn't obvious to me what these were when writing a lint recently.

src/libsyntax/ast.rs

@@ -761,9 +761,9 @@ pub enum StmtKind {
 
     /// Expr without trailing semi-colon.
     Expr(P<Expr>),
-
+    /// Expr with a trailing semi-colon.
     Semi(P<Expr>),
-
+    /// Macro.
     Mac(P<(Mac, MacStmtStyle, ThinVec<Attribute>)>),
 }