net: document ParseIP always returns IPv6 addresses

net: document ParseIP always returns IPv6 addresses

Fixes #65131

Change-Id: I050c6459a8e0a6c99425759d3131cf775b05aac9
Reviewed-on: https://go-review.googlesource.com/c/go/+/598076
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>