C++: Tweak qldoc for coroutines

Author: igfoo

cpp/ql/src/semmle/code/cpp/exprs/Expr.qll

@@ -1270,7 +1270,7 @@ class SpaceshipExpr extends BinaryOperation, @spaceshipexpr {
 }
 
 /**
- * A C/C++ co_await expression
+ * A C/C++ `co_await` expression.
  * ```
  * co_await foo();
  * ```
@@ -1284,7 +1284,7 @@ class CoAwaitExpr extends UnaryOperation, @co_await {
 }
 
 /**
- * A C/C++ co_yield expression
+ * A C/C++ `co_yield` expression.
  * ```
  * co_yield 1;
  * ```

cpp/ql/src/semmle/code/cpp/stmts/Stmt.qll

@@ -663,7 +663,7 @@ class LabelStmt extends Stmt, @stmt_label {
 }
 
 /**
- * A C/C++ 'co_return' statement.
+ * A C/C++ `co_return` statement.
  *
  * For example:
  * ```