Description Bug and Workaround
Many users have reported when adding a description to a photo or selected photos in a window that all photos received the same description. This is because of a bug in the current system and Photos.
User kkkttt found that it happened only when he didn't use the Enter key after typing in the description but click somewhere in the window (Re: Photos info description has a glitch) That would add the description to all of the photos in that window.
Using the Enter key or the Return key will limit the description to just the selected photos.
However, in order to keep the other photos in the library safe from error I suggest putting the selected photos in a new album and change the descriptions there. Then in case one forgets to use either of those two keys the rest of the library is protected.
I was able to confirm this bug and kkkttt's workaround with Safari 10.12.3 and Photos 2.0.