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>

Author: Jay Conrod

src/cmd/go/alldocs.go

@@ -38,7 +38,7 @@ Generate runs commands described by directives within existing
 files. Those commands can run any process but the intent is to
 create or update Go source files.
 
-Go generate is never run automatically by go build, go get, go test,
+Go generate is never run automatically by go build, go test,
 and so on. It must be run explicitly.
 
 Go generate scans the file for directives, which are lines of

src/cmd/go/internal/generate/generate.go

@@ -83,8 +83,7 @@ current directory. For example:
 
 See 'go help install' or https://golang.org/ref/mod#go-install for details.
 
-In addition to build flags (listed in 'go help build') 'go get' accepts the
-following flags.
+'go get' accepts the following flags.
 
 The -t flag instructs get to consider modules needed to build tests of
 packages specified on the command line.
@@ -99,6 +98,10 @@ but changes the default to select patch releases.
 When the -t and -u flags are used together, get will update
 test dependencies as well.
 
+The -x flag prints commands as they are executed. This is useful for
+debugging version control commands when a module is downloaded directly
+from a repository.
+
 For more about modules, see https://golang.org/ref/mod.
 
 For more about specifying packages, see 'go help packages'.