docs: Update pure-docker upgrading docs for 3.21

Created by: efritz

Re https://github.com/sourcegraph/sourcegraph/pull/14635#discussion_r503510335

Created by: pecigonzalo

@efritz @slimsag whats the status of this one? I would like to remove it from the 3.21 milestone to the new one or close it.

Created by: slimsag

https://sourcegraph.slack.com/archives/CJX299FGE/p1603308266066600?thread_ts=1603304618.066000&channel=CJX299FGE&message_ts=1603308266.066600

Created by: pecigonzalo

Closed in: !15412 (merged)