Update sdk_ua_app_id docs (#3019)

Author: alextwoods

gems/aws-sdk-core/lib/aws-sdk-core/plugins/user_agent.rb

@@ -14,8 +14,9 @@ class UserAgent < Seahorse::Client::Plugin
         doc_type: 'String',
         docstring: <<-DOCS) do |cfg|
 A unique and opaque application ID that is appended to the
-User-Agent header as app/<sdk_ua_app_id>. It should have a
-maximum length of 50.
+User-Agent header as app/sdk_ua_app_id. It should have a
+maximum length of 50. This variable is sourced from environment
+variable AWS_SDK_UA_APP_ID or the shared config profile attribute sdk_ua_app_id.
         DOCS
         app_id = ENV['AWS_SDK_UA_APP_ID']
         app_id ||= Aws.shared_config.sdk_ua_app_id(profile: cfg.profile)