Specify how shorthand directive options are parsed

Is your feature request related to a problem? Please describe.

The parameterizing directives section states that:

• Directives may have YAML frontmatter, parsed by the YAML formatter
• Directives may have shorthand :options: instead

It is never specified how the shorthand options are parsed. Is

```test
:a: |
  yaml-formatted multiline text
```

correct?

A link to the documentation page where you see an issue.

https://myst-parser.readthedocs.io/en/latest/using/syntax.html#parameterizing-directives

Describe the solution you’d like

I would prefer it if the documentation specified how the values of the shorthand directive options are parsed.

[akhmerov]

Ah, I see. so each line must start with a :, and not only those with the option name. I was confused by the similarity between the shorthand syntax and the rst options formatting.

[choldgraf]

Ah yeah it’s the same parsing. The shorthand is just the : leading each line to detect the yaml ok the other side, but isn’t part of the yaml itself. Eg without the short hand there are some examples of raw multi line yaml.

Want to make a PR adding this context to the place(s) where you think it’d naturally be?

Here’s the code where it is parsed https://github.com/executablebooks/MyST-Parser/blob/4b7f571addbc03624a34df8d0e496253d343a7e0/myst_parser/parse_directives.py#L114