Imply boulder and pebble to rock

BUR #19794 has been rejected.

create implication boulder -> rock
create implication pebble -> rock

Both are rocks of different sizes.

This bulk update request is pending automatic rejection in 5 days.

This bulk update request has been rejected because it was not approved within 45 days.

The bulk update request #19794 (forum #253552) has been rejected by @DanbooruBot.

Per topic #14476, post #6676784 would be stone, not rock.