cmd/go: document in 'go help mod' that people should use 'go get'

We're starting to see tutorials that assume
'go mod' is the only module-related command.

Fixes #26597.

Change-Id: I44701f29f89fc67086f96307afbdb4659bb63873
Reviewed-on: https://go-review.googlesource.com/125935
Reviewed-by: Bryan C. Mills <[email protected]>

Author: rsc

src/cmd/go/alldocs.go

@@ -28,6 +28,11 @@ var CmdMod = &base.Command{
 Mod performs module maintenance operations as specified by the
 following flags, which may be combined.
 
+Note that support for modules is built into all the go commands,
+not just 'go mod'. For example, day-to-day adding, removing, upgrading,
+and downgrading of dependencies should be done using 'go get'.
+See 'go help modules' for an overview of module functionality.
+
 The -v flag enables additional output about operations performed.
 
 The first group of operations provide low-level editing operations