Serhiy Storchaka email@example.com added the comment:
I think they are not required, but recommended.
You should use two spaces after a sentence-ending period in multi- sentence comments, except after the final sentence.
You must adhere to the Emacs convention of adding two spaces at the end of every sentence.
AFAIK in English typography the space after a sentence-ending period is longer than spaces between words. In other European typographies they have the same width.