Fix minor mistakes in the bandwidth-file dir-spec entry

atagar pointed out the following errors in the bandwidth-file dir-spec entry:

The bandwidth-file line appears:

[At most once for votes; does not occur in consensuses.]

He also suggested that we could change the name to bandwidth-file-headers or bandwidth-headers if we want. I don't mind what we call it, but it has to match #3723 (moved).

I also noticed that the definition of Value is wrong, it should be:

Value ::= ArgumentCharValue+
ArgumentCharValue ::= any printing ASCII character except NL and SP.

We should also add the new file_created and latest_bandwidth header keywords from bandwidth-file-spec.

Edit: file_created and latest_bandwidth were added in #26155 (moved) and it has been merged 3 now to master Edit: previous line is about adding them to dir-spec