add missing library descriptions to mandoc

[pstef]

Correct me if I'm wrong but since we're downstream, we want both this and D23944 to get pushed upstream first?

[bapt]

We don't need to wait for upstream to validate to get that committed into contrib, but yes we should discuss with upstream about incorporating them.

[imp]

In D13408#729640, @bapt wrote:

We don't need to wait for upstream to validate to get that committed into contrib, but yes we should discuss with upstream about incorporating them.

I concur. If there's a substantial risk that they would balk, that would be different. And if they do it differently, that's a matter for the next import.