The alias got setup by T100349 and, If I remember well, the alias docs was directed to the text cache, then to Mediawiki Apache and the redirect handled by those servers with:
funnel docs.wikimedia.org https://doc.wikimedia.org
funnel stands for:
Redirect a whole domain (or set of domains) to a single URL
So doc.wikimedia.org is the canonical URL, and docs is merely a convenience redirect for the rare case when some writes docs.wikimedia.org in the URL bar.
Eventually I found out we have hits recorded since January 4th 2018 in Pivot. Looking at https://turnilo.wikimedia.org/#webrequest_sampled_128 sampled webrequests on pivot:
From the two sampled requests we had: