docs: fix generate usage for Unreal bindings

[clockwork-labs-bot]

Summary

• Fix the spacetime generate usage string so Unreal-specific flags are no longer shown as mutually exclusive with module source flags.
• Update the current CLI reference to match the corrected usage.

Audit context

Daily skills/docs consistency audit found this mismatch between skills/unreal/SKILL.md, skills/cli/SKILL.md, and the public CLI reference: Unreal generation uses --uproject-dir and --unreal-module-name together, often with a module source flag, but the docs usage line grouped those flags as alternatives.

Validation

• git diff --check
• corepack pnpm -C docs typecheck
• corepack pnpm -C docs build

Note: cargo/rustup are not installed in this runtime, so I could not run Rust formatting or CLI tests locally.

[clockwork-labs-bot]

Consolidated this into #5166 so the docs audit fixes stay in one reviewable PR.