docs: expand and update documentation on use_execroot_entry_point

[acozzette]

---

Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: no

Test plan

[aspect-workflows]

✨ Aspect Workflows Tasks

🔄 1 in progress task

• 🔄 test (test-e2e-npm_translate_lock-bazel-9-no-execroot-entry-point) · ⏱ 1.6s · 🐙 GitHub Actions · ☑️ Check
  💬 Running setup...

---

_{⏱ Last updated Wed Jun 3 16:06:41 UTC 2026 · 📊 GitHub API quota 3,564/15,000 (24% used, resets in 31m, throttle 10×)
🚀 Powered by Aspect CLI (v2026.22.44)  |  Aspect Build · X · LinkedIn · YouTube}