Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
3$ nailing-cargo <nailing-opts> --- [--] <build-command>...
```
Ususally the `--` is not needed. (It should generally be passed by
-programs which wrap nailing-cargo. See [Invocation argument disambiguation rules], below.)
+programs which wrap nailing-cargo. See [Invocation argument disambiguation rules](#invocation-argument-disambiguation-rules), below.)
In usage 1, nailing-cargo runs `cargo` (from `PATH`). In the usage 2
nailing-cargo runs `<cargo>`. In both these cases it adds its own