Fixes for -listing-visibility flag values in CLI tools (#15838)

See also: #15833, #15209

Signed-off-by: Brian Candler <b.candler@pobox.com>

website/content/docs/commands/auth/enable.mdx

@@ -69,7 +69,9 @@ flags](/docs/commands) included on all commands.
   this auth method.
 
 - `-listing-visibility` `(string: "")` - The flag to toggle whether to show the
-  mount in the UI-specific listing endpoint.
+  mount in the UI-specific listing endpoint. Valid values are `"unauth"` or `"hidden"`,
+  with the default `""` being equivalent to `"hidden"`.
+
 
 - `-local` `(bool: false)` - Mark the auth method as local-only. Local auth
   methods are not replicated nor removed by replication.

website/content/docs/commands/auth/tune.mdx

@@ -68,7 +68,8 @@ flags](/docs/commands) included on all commands.
   This overrides the current stored value, if any.
 
 - `-listing-visibility` `(string: "")` - The flag to toggle whether to show the
-  mount in the UI-specific listing endpoint.
+  mount in the UI-specific listing endpoint. Valid values are `"unauth"` or `"hidden"`.
+  Passing empty string leaves the current setting unchanged.
 
 - `-max-lease-ttl` `(duration: "")` - The maximum lease TTL for this auth
   method. If unspecified, this defaults to the Vault server's globally

website/content/docs/commands/secrets/tune.mdx

@@ -75,7 +75,8 @@ flags](/docs/commands) included on all commands.
   This overrides the current stored value, if any.
 
 - `-listing-visibility` `(string: "")` - The flag to toggle whether to show the
-  mount in the UI-specific listing endpoint.
+  mount in the UI-specific listing endpoint. Valid values are `"unauth"` or `"hidden"`.
+  Passing empty string leaves the current setting unchanged.
 
 - `-max-lease-ttl` `(duration: "")` - The maximum lease TTL for this secrets
   engine. If unspecified, this defaults to the Vault server's globally