How do I provide search of a specific document library on my content page?

Hello Vanessa Martin9 

you need to configure your results web part with a specific path for your results. In the query, also in classic, you can add a search results and change the query like this:

The current path:

{searchboxquery} Path:{site.url}

Or a specific path and only documents:

{searchboxquery} Path:"https://TENANT.sharepoint.com/sites/SPOClassicEnterpriseWiki/Documents/" IsDocument:1

Regards, Dave