Add "auth help" command documentation

website/source/docs/commands/auth/help.html.md

@@ -0,0 +1,45 @@
+---
+layout: "docs"
+page_title: "auth help - Command"
+sidebar_current: "docs-commands-auth-help"
+description: |-
+  The "auth help" command prints usage and help for an auth method.
+---
+
+# auth help
+
+The `auth help` command prints usage and help for an auth method.
+
+  - If given a TYPE, this command prints the default help for the auth method of
+    that type.
+
+  - If given a PATH, this command prints the help output for the auth method
+    enabled at that path. This path must already exist.
+
+Each auth method produces its own help output.
+
+## Examples
+
+Get usage instructions for the userpass auth method:
+
+```text
+$ vault auth help userpass
+Usage: vault login -method=userpass [CONFIG K=V...]
+
+  The userpass authentication method allows users to authenticate using Vault's
+  internal user database.
+
+# ...
+```
+
+Print usage for the auth method enabled at my-method/
+
+```text
+$ vault auth help my-method/
+# ...
+```
+
+## Usage
+
+There are no flags beyond the [standard set of flags](/docs/commands/index.html)
+included on all commands.