docs: Update Cargo workspace support (#13349)
## Why
The Cargo workspace future flag still claimed task execution and caching
were unwired, which no longer matches the implementation.
## What
The authoritative flag descriptions now explain Cargo package discovery,
synthesized task execution, library no-op semantics, and derived caching
while retaining the experimental status.
## How
Updated the Rust source description and schema generator, then
regenerated and verified the JSON Schema and TypeScript outputs.
