Update docs

contracts/access/AccessControl.sol

@@ -19,14 +19,14 @@ import "../utils/introspection/ERC165.sol";
  * in the external API and be unique. The best way to achieve this is by
  * using `public constant` hash digests:
  *
- * ```
+ * ```solidity
  * bytes32 public constant MY_ROLE = keccak256("MY_ROLE");
  * ```
  *
  * Roles can be used to represent a set of permissions. To restrict access to a
  * function call, use {hasRole}:
  *
- * ```
+ * ```solidity
  * function foo() public {
  *     require(hasRole(MY_ROLE, msg.sender));
  *     ...
@@ -44,7 +44,8 @@ import "../utils/introspection/ERC165.sol";
  *
  * WARNING: The `DEFAULT_ADMIN_ROLE` is also its own admin: it has permission to
  * grant and revoke this role. Extra precautions should be taken to secure
- * accounts that have been granted it.
+ * accounts that have been granted it. We recommend using {AccessControlDefaultAdminRules}
+ * to enforce additional security measures for this role.
  */
 abstract contract AccessControl is Context, IAccessControl, ERC165 {
     struct RoleData {