fixes stated grammar for block comments, by adding Kleene '+' in two places.

Closes #1588
fix another subtle bug in comment regex
added defn of non_slash_or_star
auto merge of #5583 : jbclements/rust/docfix-for-block-comment-grammar, r=jbclements

... by adding Kleene '+' in two places, and changing a "non-slash" into "non_slash_or_star".

Closes #1588