defaultStorageSettings() and defaultFieldSettings() should document that they must not have setting names in common

Problem/Motivation

These two methods in FieldItemInterface look like they're totally separate, but other parts of core assume that the setting names are distinct.

For example in BaseFieldDefinition the two arrays are merged:

    $default_settings = $field_type_manager->getDefaultStorageSettings($type) + $field_type_manager->getDefaultFieldSettings($type);

Steps to reproduce

Proposed resolution

Document in both methods that setting names must be different from the ones returned by the other method.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet