Add information about specific things in Fragment

[Fedoseew]

Add to fragment documentation block about fragment specific.
For example when we want to find component inside fragment by component ID we should use special method FragmentUtils#getComponentId, not method from UiComponentUtils

[knstvk]

@sukhova - plz find out the whole story and add required info to https://docs.jmix.io/jmix/flow-ui/fragments/fragments.html#fragment-api

[iudaev]

Also mention that components within fragments use hidden ids. "Components inside the fragment are not assigned actual IDs. Hidden IDs are set for them, and FragmentUtils is used to work with them." See https://demo.jmix.io/ui-samples/sample/fragment-simple?tab=Description