Clarify documentation of markdown parsing in metadata files

[calliecameron]

The documentation for --metadata-file in the manual says: 'Read metadata from the supplied YAML (or JSON) file. This option can be used with every input format, but string scalars in the YAML file will always be parsed as Markdown'.

I read this as 'the file can be YAML or JSON, but string scalars will only be parsed as markdown in YAML files, not in JSON files'. In fact they're parsed as markdown in both types of files. The documtation should make this clear.

[tarleb]

Thank you for raising the issue. If you already have something in mind, please let us know. Suggestion for alternative phrasing are always welcome.

[calliecameron]

How about: 'Read metadata from the supplied YAML or JSON file. This option can be used with every input format, but string scalars in the metadata file will always be parsed as Markdown.'

[jgm]

Thanks! I made the change.