command/auth, github: improve cli docs

/cc @sethvargo

builtin/credential/github/cli.go

@@ -43,6 +43,12 @@ account settings page on GitHub.
 
     Example: vault auth -method=github token=<token>
 
+Key/Value Pairs:
+
+    mount=github      The mountpoint for the GitHub credential provider.
+	                  Defaults to "github"
+
+    token=<token>     The GitHub personal access token for authentication.
 	`
 
 	return strings.TrimSpace(help)

command/auth.go

@@ -242,7 +242,13 @@ Usage: vault auth [options] [token or config...]
   By specifying -method, alternate authentication methods can be used
   such as OAuth or TLS certificates. For these, additional values for
   configuration can be specified with "key=value" pairs just like
-  "vault write".
+  "vault write". Specify the "-method-help" flag to get help for a specific
+  method.
+
+  If you've mounted a credential backend to a different path, such
+  as mounting "github" to "github-private", the "method" flag should
+  still be "github." Most credential providers support the "mount" option
+  to specify the mount point. See the "-method-help" for more info.
 
 General Options: