Mention that custom servlet is registered as sub-path of web application context path

[alexbudarov]

Environment

• Existing pages: https://doc.cuba-platform.com/manual-7.1/servlet_registration_sample.html https://doc.cuba-platform.com/manual-7.1/servlet_registration.html

Description of the bug or enhancement

In the documentation page it's not mentioned that custom servlet will be available under "/app/webd/" or "/app-core/webd/" context path, prepended by web application name.

See this forum topic with confusion: https://www.cuba-platform.ru/discuss/t/nastrojka-websocket/3940/6