cmd/go: document GO111MODULE=auto behavior for Go 1.13

Fixes #31857

Change-Id: Ic4afe1262d863112db97794edc1ade20f6926d64
Reviewed-on: https://go-review.googlesource.com/c/go/+/180197
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modload/help.go

@@ -21,23 +21,26 @@ which source files are used in a given build.
 
 Module support
 
-Go 1.13 includes official support for Go modules,
-including a module-aware 'go get' command.
-Module-aware mode is active by default.
+Go 1.13 includes support for Go modules. Module-aware mode is active by default
+whenever a go.mod file is found in, or in a parent of, the current directory.
+
+The quickest way to take advantage of module support is to check out your
+repository, create a go.mod file (described in the next section) there, and run
+go commands from within that file tree.
 
 For more fine-grained control, Go 1.13 continues to respect
 a temporary environment variable, GO111MODULE, which can be set to one
-of three string values: off, auto, or on (the default).
-If GO111MODULE=on or is unset, then the go command requires the use of
-modules, never consulting GOPATH. We refer to this as the command
+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=auto, then the go command enables or disables module
-support based on the current directory. Module support is enabled only
-when the current directory is outside GOPATH/src and itself contains a
-go.mod file or is below a directory containing a go.mod file.
 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.
+Module support is enabled only when the current directory contains a
+go.mod file or is below a directory containing a go.mod file.
 
 In module-aware mode, GOPATH no longer defines the meaning of imports
 during a build, but it still stores downloaded dependencies (in GOPATH/pkg/mod)