Find identity providers by brand name when unique.

Signed-off-by: Jason Volk <jason@zemos.net>

tuwunel-example.toml

@@ -2141,8 +2141,13 @@
 # for your convenience. For certain brands we apply essential internal
 # workarounds specific to that provider; it is important to configure this
 # field properly when a provider needs to be recognized (like GitHub for
-# example). Several configured providers can share the same brand name. It
-# is not case-sensitive.
+# example).
+#
+# Several configured providers can share the same brand name. It is not
+# case-sensitive. As a convenience for common simple deployments we can
+# identify this provider by brand in addition to the unique `client_id` if
+# and only if there is a single provider for the brand; see notes for
+# `client_id`.
 #
 #brand =
 
@@ -2151,6 +2156,11 @@
 # instance itself, becoming the identity provider's ID and must be unique
 # and remain unchanged.
 #
+# As a convenience we also identify this config by `brand` if and only if
+# there is a single provider configured for a `brand`. Note carefully that
+# multiple providers configured with the same `brand` is not an error and
+# this provider will simply not be found when querying by `brand`.
+#
 #client_id =
 
 # Secret key the provider generated for you along with the `client_id`