Add a link to OAuth 2 docs in help texts (#4034)

* Update installed_oauth_apps.jsx

* Update custom_integrations_settings.jsx

* Update confirm_integration.jsx

* Update installed_oauth_apps.jsx

* Update en.json

* Change FormattedMessage to FormattedHTMLMessage

* Fix proper coding of links in en.json

* Update en.json

webapp/components/admin_console/custom_integrations_settings.jsx

@@ -115,9 +115,9 @@ export default class WebhookSettings extends AdminSettings {
                         />
                     }
                     helpText={
-                        <FormattedMessage
+                        <FormattedHTMLMessage
                             id='admin.oauth.providerDescription'
-                            defaultMessage='When true, Mattermost can act as an OAuth 2.0 service provider allowing Mattermost to authorize API requests from external applications.'
+                            defaultMessage='When true, Mattermost can act as an OAuth 2.0 service provider allowing Mattermost to authorize API requests from external applications. See <a href="https://docs.mattermost.com/developer/oauth-2-0-applications.html" target="_blank">documentation</a> to learn more.'
                         />
                     }
                     value={this.state.enableOAuthServiceProvider}