cmd/cue: add a reference to the Central Registry from cue help modules

The `cue help modules` text does not mention the central registry
directly. Change the docs to include a link to something explaining how
to work it.

Fixes #3202

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Iac942507a2497e10b386008e9abdd773bc259b4a
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198081
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cmd/cue/cmd/help.go

@@ -279,10 +279,18 @@ A module is a collection of packages that are released, versioned, and
 distributed together. Modules are downloaded from OCI-compliant
 artifact registries.
 
+The Central Registry for CUE modules (` + modconfig.DefaultRegistry + `) is
+used as the source for external modules by default.
+
 For a detailed reference on modules:
 
 	https://cuelang.org/docs/reference/modules/
 
+For tutorials on how to use the Central Registry, see:
+
+	https://cuelang.org/docs/tutorial/working-with-the-central-registry/
+	https://cuelang.org/docs/tutorial/publishing-modules-to-the-central-registry/
+
 For a tutorial on how to work with a custom OCI registry for CUE modules:
 
 	https://cuelang.org/docs/tutorial/working-with-a-custom-module-registry/
@@ -303,8 +311,8 @@ Given a particular registry configuration, it's possible to work what registry a
 given module is stored in, and what repository within that registry, without
 making any network calls.
 
-If no explicit registry configuration is present, a default central registry
-(` + modconfig.DefaultRegistry + `) will be used for all modules.
+If no explicit registry configuration is present, a default Central Registry
+(` + modconfig.DefaultRegistry + `) is used for all modules.
 
 The simplest way of specifying a registry configuration is to set $CUE_REGISTRY
 to the hostname of that registry.

cmd/cue/cmd/testdata/script/help_registryconfig.txtar

@@ -11,8 +11,8 @@ Given a particular registry configuration, it's possible to work what registry a
 given module is stored in, and what repository within that registry, without
 making any network calls.
 
-If no explicit registry configuration is present, a default central registry
-(registry.cue.works) will be used for all modules.
+If no explicit registry configuration is present, a default Central Registry
+(registry.cue.works) is used for all modules.
 
 The simplest way of specifying a registry configuration is to set $CUE_REGISTRY
 to the hostname of that registry.