docs: Line number requires specified language

[matevz]

This is the behavior introduced by oasisprotocol/docs#820. This is also the same as in the official markdown code block.

e.g. This doesn't work

```{2-3}
a
b
c
d
```

but this does:

```text {2-3}
a
b
c
d
```

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[lubej]

Any reason why this changes aren't included in oasisprotocol/docs#820?

[matevz]

Any reason why this changes aren't included in oasisprotocol/docs#820?

You probably meant https://github.com/oasisprotocol/cli/pull/238/files#diff-f47f6c25f8613e21081f4dd5cbbcdd07aaa87b7ebf8898b1ddfbde86cfedd1c5R43 ? Because the initial implementation of ![code {4}](snippet.out) didn't actually work, but my tests used a cached version of the docs and thought it worked when merging it.