cmd/go: adjust documentation mentioning 'go get'

In module-aware mode, 'go get' no longer builds or installs packages.

- 'go generate' explains build commands do not run generate
  commands. 'go get' is no longer a build command, so this CL removes
  mention of it.
- 'go get' will continue to accept build flags, but they're
  ignored. The documentation no longer mentions them, though it does
  mention -x for printing VCS commands.

For #43684

Change-Id: I1eea7241eecf72ba9f98238b729d91cc77ec7665
Reviewed-on: https://go-review.googlesource.com/c/go/+/355209


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>