Extend description of inst command in dracut.modules man page

[jougs]

This pull request extends the description of the inst command to mention that the filename has to be present in the <dst> given and that directories in <dst> will be created if they don't exist yet.

Background: When omitting the filename from <dst>, only the first call to inst seems to succeed, while subsequent calls are skipped without an error. Feel free to accept this PR in case this is the intended behavior. If the current behavior is considered a bug, it might be better to fix it and adapt the manpage accordinly.

Checklist

• I have tested it locally
• I have reviewed and updated any documentation if relevant
• I am providing new code and test(s) for it