Lowdown doesn't quite conform to CommonMark in parsing shortcut links
that are followed by a parenthesized expression, which looks like
`[link text] (unrelated text)`. CommonMark says the space there is
significant and ensures the `[link text]` is parsed as a shortcut link,
but Lowdown parses this like `[link text](unrelated text)`.
This fixes the output of `nix help`. The other case of a near-link was
in the `nix-env --install` docs, which don't get parsed by Lowdown, but
it turns out the link reference definition was missing. The generated
manpage stripped the brackets but the HTML manual page rendered the
broken link with brackets.
Change-Id: I6a6a69641fd2dbf9930bcd875ed21ea80fba909a
Lily Ballard
97f1c5cfa1