cmd/go: add more docs for go directive in go.mod file

Updates #30791

Change-Id: I67efd7fd3b3a550428b16518bb27a3d81c178d28
Reviewed-on: https://go-review.googlesource.com/c/go/+/181840
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

Author: ianlancetaylor

src/cmd/go/alldocs.go

@@ -464,5 +464,12 @@ Because the module graph defines the meaning of import statements, any
 commands that load packages also use and therefore update go.mod,
 including go build, go get, go install, go list, go test, go mod graph,
 go mod tidy, and go mod why.
+
+The expected language version, set by the go directive, determines
+which language features are available when compiling the module.
+Language features available in that version will be available for use.
+Language features removed in earlier versions, or added in later versions,
+will not be available. Note that the language version does not affect
+build tags, which are determined by the Go release being used.
 	`,
 }