docs(delete-user-data): correct empty search fields description

[CorieW]

The AUTO_DISCOVERY_SEARCH_FIELDS description says "If left empty, document fields will not be used in auto discovery." That isn't what happens. The Extensions backend stores an empty value as unset and applies the id,uid,userId default.

• Confirmed on corie-testing against firebase/delete-user-data@0.1.30. A validateOnly create with AUTO_DISCOVERY_SEARCH_FIELDS: "" resolves to id,uid,userId, and so does a deployed instance configured with an empty value.
• The sentence now reads "If left empty, the default fields (id,uid,userId) are used.", as suggested in the fix(kits/delete-user-data): fall back to default search fields when empty #3231 review. Changed in extension.yaml and the generated README.md.
• Docs only, so no version bump. The new text ships with the next release.
• The kit change is in its own PR into kits, because next has no kits/ folder.

[gemini-code-assist]

Code Review

This pull request updates the documentation in README.md and the parameter description in extension.yaml for the 'Auto discovery search fields' configuration. It clarifies that if the field is left empty, the default fields (id,uid,userId) are used instead of auto-discovery not being used. There are no review comments, and I have no additional feedback to provide.