Yes we can, but we "don't modify a post such that it changes the meaning of the post."
Usually a mark is left after editing the post. This can be red or in a small font (Edit: reason..), but it's not mandatory. Moderators are not restricted in this regard.

About the post you showed as an example only Xal0lex, who made the edit, can answer though. I don't know what changes he made in the post.