cmd/cue: update mod docs

The `cue mod tidy` command didn't have an intro,
and `cue mod publish` did not mention `$CUE_REGISTRY`.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I1dc7ce7cad420bcb83b8dcc6eb3f816481c3442a
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1176758
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@gmail.com>

cmd/cue/cmd/modpublish.go

@@ -34,8 +34,14 @@ func newModUploadCmd(c *Command) *cobra.Command {
 		Short: "publish the current module to a registry",
 		Long: `WARNING: THIS COMMAND IS EXPERIMENTAL.
 
-Publish the current module to an OCI registry.
-Also note that this command does no dependency or other checks at the moment.
+Publish the current module to an OCI registry. It consults
+$CUE_REGISTRY to determine where the module should be published (see
+"cue help environment" for details). Also note that this command does
+no dependency or other checks at the moment.
+
+Note: you must enable the modules experiment with:
+	export CUE_EXPERIMENT=modules
+for this command to work.
 `,
 		RunE: mkRunE(c, runModUpload),
 		Args: cobra.ExactArgs(1),

cmd/cue/cmd/modtidy.go

@@ -33,7 +33,21 @@ func newModTidyCmd(c *Command) *cobra.Command {
 		Short: "download and tidy module dependencies",
 		Long: `WARNING: THIS COMMAND IS EXPERIMENTAL.
 
-Currently this command must be run in the module's root directory.
+Tidy resolves all module dependencies in the current module and updates
+the cue.mod/module.cue file to reflect them.
+
+It also removes dependencies that are not needed.
+
+It will attempt to fetch modules that aren't yet present in the
+dependencies by fetching the latest available version from
+a registry.
+
+See "cue help environment" for details on how $CUE_REGISTRY is used to
+determine the modules registry.
+
+Note: you must enable the modules experiment with:
+	export CUE_EXPERIMENT=modules
+for this command to work.
 `,
 		RunE: mkRunE(c, runModTidy),
 		Args: cobra.ExactArgs(0),