I believe you should record what you've found. By not reporting your findings will not make the document go away, and it will remain there for another family member to find when they take up genealogy. There's a difference between altering and omitting an item found in your research, but omitting something sometimes alters the context of what you write.
You might consider looking for a probate court record - probate court is what it's called in Michigan - to see if your 1880 relative was declared legally insane for the purpose of someone being able to administer his affairs and property as a guardian. There is a lot to be read into this action.