net: improve documentation of netgo/netcgo build tags
Updates #69680
Change-Id: If80d43da493cb035164ede4a2cadcbec5831feaf
GitHub-Last-Rev:
b3853e9487da1006ecece9dd378e3de299d0b1da
GitHub-Pull-Request: golang/go#69682
Reviewed-on: https://go-review.googlesource.com/c/go/+/616262
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>