docs: clarify accessible name precedence

[wanxiankai]

Summary

Clarify how the name option in *ByRole queries computes an accessible name in RNTL v14.

The docs now list the precedence of labelled-by references, explicit accessibility labels, and fallback content such as image alt text, a root text input placeholder, or child text. They also include an example showing that an explicit accessibilityLabel takes precedence over a button's child text.

Closes #1923.

Test plan

• yarn docs:generate
• yarn --cwd website validate
• yarn typecheck
• yarn test --watchman=false (83 suites, 795 tests, and 236 snapshots passed)
• yarn lint
• yarn format:check
• yarn build

yarn validate reached Jest but Watchman could not write its macOS LaunchAgent in the sandbox. The complete Jest suite was rerun with Watchman disabled as shown above; the remaining validation commands were run separately and passed.