vim: disable nested comment

Since comment nesting does not work from 0.4.
vim: add rustCommentDoc
vim: fix comment highlighting bug

Previous commit had a bug that a line which ends with "//" or "/*"
is not correctly highlighted.
auto merge of #5760 : klutzy/rust/vim, r=thestinger

1. disable nested comment which is not supported now
2. add syntax highlight for rustCommentDoc e.g. `/** doc */` or `/// doc`.