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

[stale]

This issue is being marked as stale because it has not had any recent activity. It will be closed if no further activity occurs. If this is still an issue in the latest release of Dracut and you would like to keep it open please comment on this issue within the next 7 days. Thank you for your contributions.

[jougs]

Sorry, I seem to have missed the comment by stalebot above. Anyway: I think this deserves to be re-opened. Or shall I create a new PR from the same branch instead? Thanks!