docs: Note of hidden properties

[neznaika0]

Description
See #9748

1. Why doesn't the documentation describe the case of underlining properties? They are skipped when calling Entity::toArray() but allowed in Entity::toRawArray()
2. The behavior of superfluous properties is not described. How they will be shown for the entity and when working with the DB.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[neznaika0]

@michalsn, updated. Can we add something related to the database? How can this be useful or bad?

[michalsn]

Looks good, thanks!