Print note when trying to use unstable --print target-spec-json

#38061 introduced `--print target-spec-json` but it's a little hard to find out how to use it:

Current:
```
$ rustc +nightly --print target-spec-json
error: unknown print request `target-spec-json`
```
Expected:
```
$ rustc +nightly --print target-spec-json
error: the `-Z unstable-options` flag must also be passed to enable the print request `target-spec-json`
```