netdoc metadescription BNF is not universally applicable

dir-spec/outline.md has a BNF for a netdoc item line.

But this is a lie. This BNF is not always used.

In particular it doesn't discuss items with a "rest of line" argument, for example contact in a server descriptor. There are other things that are confusing about it, some of which are discussed in !305 (comment 3122170)

This situation should be remedied and the section tidied up more generally.