The guidelines of editing is already in the editing page, and also have been blogged before:
As it says on the sidebar of every edit page, here’s what makes up good editing practice as we see it on Stack Overflow:
Fix grammatical or spelling errors.
Clarify meaning without changing it.
Correct minor mistakes.
Add related resources or links.
Always respect the original author.
You edit to make things better, clearer, more effective — never to change meaning.
While Bill's addendum does clarify mistakes of the poster, I would not encourage this kind of editing as this does alter the original theme of the answer.