User guide: drop/rework comment about GA params

[chalin]

Originally posted by @chalin in https://github.com/google/docsy/issues/1098#issuecomment-1185659042

... referring the reader to the appropriate section of the Hugo docs. The page update includes a warning (about not setting both googleAnalytics and services.googleAnalytics.id), but eventually (once https://github.com/gohugoio/hugoDocs/issues/1780 is addressed), we'll be able to remove the warning.