docs: clarify ruleset pass-through and supported target types

[joshjohanning]

Clarifies two things about ruleset syncing that weren't obvious from the existing docs:

• The action passes ruleset JSON through to the GitHub Rulesets API as-is (with read-only fields like id, source, source_type, created_at, etc. stripped automatically). That means new rule types and parameters GitHub adds, such as copilot_code_review.parameters.review_draft_pull_requests, sync without needing an action update.
• All target values the API supports work, not just branch. Calls out branch (default), tag, and push, and recommends setting target explicitly in each ruleset JSON file.

README-only change; existing examples already include "target": "branch" so no example edits were needed.

[Copilot]

Pull request overview

This PR updates the README documentation for the action’s repository ruleset syncing feature, clarifying how ruleset JSON is handled and what target values are supported.

Changes:

• Document that ruleset JSON is effectively passed through to the Rulesets API, with known read-only API fields stripped automatically.
• Clarify that all API-supported ruleset target types are supported (including branch, tag, and push) and recommend setting target explicitly.

Show a summary per file

File	Description
README.md	Clarifies ruleset pass-through behavior and supported target values for ruleset sync.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0