help: add doc for GO111MODULE.

Fixes golang#43419

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

@@ -535,6 +535,14 @@ General-purpose environment variables:
 	GOTMPDIR
 		The directory where the go command will write
 		temporary source files, packages, and binaries.
+	GO111MODULE
+		which can be set to one of three string values: off, on, or auto (the default).
+		If GO111MODULE=on, then the go command requires the use of modules, never consulting GOPATH. 
+		We refer to this as the command being module-aware or running in "module-aware mode".
+		If GO111MODULE=off, then the go command never uses module support. 
+		Instead it looks in vendor directories and GOPATH to find dependencies; we now refer to this as "GOPATH mode."
+		If GO111MODULE=auto or is unset, then the go command enables or disables
+		module support based on the current directory.
 
 Environment variables for use with cgo: