If you ever missed a full-featured spell-and-grammar checker on iOS then you should check out Grammarly. Grammarly is a replacement keyboard for iOS which not only offers auto-correction as you type, but can also run through the entire text and suggest changes, just like you can do on the Mac.
But Grammarly goes one better than that. Its grammar engine picks up on errors of punctuation, grammar, and even spots double spaces. I just ran it on the previous article I wrote today and it flagged up all kinds of small errors that a regular spell checker would never catch.