fix mrb_notimplement typo
Fix mailing-list description
Merge pull request #2637 from rmosolgo/not-implemented-error-msg

fix mrb_notimplement typo
Merge pull request #2639 from suzukaze/fix-mailing-list-description

Fix mailing list description; ref #2635
[ci skip] rephrasing about mailing list description; ref #2639
separate mrb_notimplement() and mrb_notimplement_m(); ref #2636