Say I were to write an opinionated letter to the editor. Then, the newspaper edited my article for content (deleted something they didn't like, changed wording, or otherwise changed the meaning of my letter). Would it be legal for them to publish this under my name, without a disclaimer that they had edited it?
Newspapers typically have some public statement to the effect that they reserve the right to edit letters to the editor, though you may have to dig for it. Some papers inform authors of rewordings in advance, and most (good) papers will publish something resembling a retraction if they mistakenly pervert your argument through editing. You permission is probably obtained via a copyright permission form or click-through, and legality basically comes down to your consent. If you write an letter in favor of A which they rewrite into opposition to A, that would not be libel. It is very unlikely that a paper would edit a letter to introduce libel (e.g. changing "The Mayor helped these children" into "The Mayor raped these children"), but if they did, you would not be liable for the libel (since it wasn't you who wrote that). There is no general requirement that publishers include disclaimers that items have been edited: publishers universally edit, and authors live with it, or insist on no-editing (which effectively means no-publishing, for letters).