The current rule for sentence ending is: Sure, it's arguable, but I don't want to change it, partly because I think Knuth made the right decision, partly because there's no way to do things at the "word" level in TeX, and partly because it would introduce a significant incompatibility in Texinfo with only a minor benefit.
There have always been commands to Texinfo to explicitly specify whether a period ends a sentence or not, and using those is (and always has been) the way to get the correct output. Best, karl
