[UIK-4893][website] added some notes about how ellipsis works

[ilyabrower]

Motivation and Context

I've just added some notes about children observers in Ellipsis

How has this been tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• Nice improve.

Checklist:

• I have updated the documentation accordingly.
• I have added new tests on added of fixed functionality.

[sheila-semrush]

@ilyabrower I thought it will be more clear if we make a separate section out of it and add an example, if you don't mind

left your original text, just with some slight fixes