docs: clarify invalid pagination value handling

[tamjid14]

Summary

This PR clarifies how invalid pagination values are handled in the README.

It adds a short note explaining that invalid _page or _per_page values are normalized to valid ranges, with _per_page=0 as a concrete example.

Why

This makes the pagination behavior clearer for users who may expect _per_page=0 to return zero items.

Testing

• Documentation-only change
• No source code changes