cmd/cue: un-hide login command and update docs

We will soon publish the first tutorials using the central registry,
so they will ask users to run `cue login`.
Un-hide the command so it can be found with its documentation.

Tweak the documentation after the $CUE_CONFIG_DIR change as well.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I5dcd754e89dacf815d4460725c9aa72793b76765
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1185185
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

cmd/cue/cmd/login.go

@@ -46,19 +46,15 @@ import (
 
 func newLoginCmd(c *Command) *cobra.Command {
 	cmd := &cobra.Command{
-		// TODO: this command is still experimental, don't show it in
-		// the documentation just yet.
-		Hidden: true,
-
 		Use:   "login [registry]",
 		Short: "log into a CUE registry",
 		Long: `WARNING: THIS COMMAND IS EXPERIMENTAL.
 
 Log into a CUE registry via the OAuth 2.0 Device Authorization Grant.
 Without an argument, CUE_REGISTRY is used if it points to a single registry.
 
-Once the authorization is successful, a token is stored in a cue/logins.json file
-inside your user's config directory, such as $XDG_CONFIG_HOME or %AppData%.
+Once the authorization is successful, a token is stored in a logins.json file
+inside $CUE_CONFIG_DIR; see 'cue help environment'.
 `,
 		Args: cobra.MaximumNArgs(1),
 		RunE: mkRunE(c, func(cmd *Command, args []string) error {

cmd/cue/cmd/testdata/script/help.txtar

@@ -44,6 +44,7 @@ Available Commands:
   get            add dependencies to the current module
   help           Help about any command
   import         convert other formats to CUE files
+  login          log into a CUE registry
   mod            module maintenance
   trim           remove superfluous fields
   version        print CUE version