Searching for api methods in the documentation rarely returns useful results. I often use Google, which is much better at finding the right Anvil pages than Anvil itself. (I do the same when I search MSDN, because often neither Bing nor the MSDN search tools are able to find the correct pages in their very own MSDN).
A search like this will work: site:anvil.works/docs fetch_only.
Said that, Google will immediately take you to the only page containing the word fetch_only, which would have helped you a little, but the fetch_only method seems to be missing in the API reference docs.