[ci skip] Refine Action Mailer guide.
* wrap 80 words if possible.
* Fix incorrect create method when failed to save user:
  should `format.json { render :show ... }` instead of `format.json { render json: ... }`.
* Make attachment hash options more readable.
* L619 wrap "location" in code: `"location"`.
* L628 Use actual section name.
* L697 remove unnecessary `./` when refer to other guide.