cmd/go/internal/help: add documentation for language version downgrading

This change adds documentation for language version downgrading using
build constraints.

Fixes golang#68161
For golang#61894

Change-Id: I283a51afd7020c9fd1f5469a6a93fd814ba32f7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/595475
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/help/helpdoc.go

@@ -835,6 +835,9 @@ line comment that begins
 
  //go:build
 
+Build constraints can also be used to downgrade the language version
+used to compile a file.
+
 Constraints may appear in any kind of source file (not just Go), but
 they must appear near the top of the file, preceded
 only by blank lines and other comments. These rules mean that in Go
@@ -954,5 +957,9 @@ only when building the package for 32-bit x86.
 Go versions 1.16 and earlier used a different syntax for build constraints,
 with a "// +build" prefix. The gofmt command will add an equivalent //go:build
 constraint when encountering the older syntax.
+
+In modules with a Go version of 1.21 or later, if a file's build constraint
+has a term for a Go major release, the language version used when compiling
+the file will be the minimum version implied by the build constraint.
 `,
 }