Add documentation for Public Client PKCE support

Fixes gh-7279

Author: sdoxsee

docs/manual/src/docs/asciidoc/_includes/servlet/oauth2/oauth2-client.adoc

@@ -407,6 +407,29 @@ A request with the base path `/oauth2/authorization/okta` will initiate the Auth
 The `AuthorizationCodeOAuth2AuthorizedClientProvider` is an implementation of `OAuth2AuthorizedClientProvider` for the Authorization Code grant,
 which also initiates the Authorization Request redirect by the `OAuth2AuthorizationRequestRedirectFilter`.
 
+If the OAuth 2.0 Client is a https://tools.ietf.org/html/rfc6749#section-2.1[Public Client], than configure the OAuth 2.0 Client registration as follows:
+
+[source,yaml]
+----
+spring:
+  security:
+    oauth2:
+      client:
+        registration:
+          okta:
+            client-id: okta-client-id
+            client-authentication-method: none
+            authorization-grant-type: authorization_code
+            redirect-uri: "{baseUrl}/authorized/okta"
+            ...
+----
+
+Public Clients are supported using https://tools.ietf.org/html/rfc7636[Proof Key for Code Exchange] (PKCE).
+If the client is running in an untrusted environment (eg. native application or web browser-based application) and therefore incapable of maintaining the confidentiality of it's credentials, PKCE will automatically be used when the following conditions are true:
+
+. `client-secret` is omitted (or empty)
+. `client-authentication-method` is set to "none" (`ClientAuthenticationMethod.NONE`)
+
 The `DefaultOAuth2AuthorizationRequestResolver` also supports `URI` template variables for the `redirect-uri` using `UriComponentsBuilder`.
 
 The following configuration uses all the supported `URI` template variables: