_{It seems that right now, in 2018, it's the default behavior.}

The link detection gets triggered when you enter a space or press enter after pasting or writing the URL.
So, if you paste a URL that has space at the end of it - it won't automatically detect it (at least not at first), and will treat it as simple text. For example "https://google.com ".

You can verify this setting under Tools --> Preferences:

When you have an HTML file in Google Drive and ask to open it with Google Docs, there's no 'convert' option AFAICT and you get a 'view source' sort of view instead of seeing it rendered.

If you have the HTML file saved locally and you upload from the Google Docs page itself, there is a 'convert' option in the upload dialog, and that will produce a rendered version of the page. In my limited experience with that conversion step (importing some HTML produced by the ASCIIDoc formatter), the rendered HTML didn't look very good -- it came out with an awful-looking font and lost some formatting from the original HTML.

The top-rated Google search results for importing HTML into Google Docs all seem to assume you're importing HTML data into a spreadsheet.

So while it seems to be possible to get some kind of rendered HTML by going through one specific path, this doesn't appear to be a high priority for Google Docs.