doc: note that FreeEnvironment() runs a shared event loop

[codebytere]

embedding.md says each node::Environment has exactly one uv_loop_t and that a node::IsolateData can be shared between Environments, and CreateIsolateData() takes the loop, so several same-thread Environments end up sharing one. What it doesn't say is that FreeEnvironment() runs that loop until the freed Environment's handles have closed, with JavaScript disallowed on the whole isolate while it does, so timers, I/O callbacks and thread pool completions belonging to the other Environments can fire inside that call and can't call into JS.

This adds a paragraph to embedding.md describing that and the two ways to stay clear of it (a loop and IsolateData per independently freed Environment, or no pending work on the siblings when one is freed), plus a pointer on FreeEnvironment() in node.h. Docs only.

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Disclosure: this change and description were written by Claude Code, directed and reviewed by @codebytere.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/76795/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.05%. Comparing base (705646f) to head (497cd51).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65691      +/-   ##
==========================================
- Coverage   90.07%   90.05%   -0.02%     
==========================================
  Files         754      754              
  Lines      256395   256395              
  Branches    48499    48507       +8     
==========================================
- Hits       230937   230898      -39     
- Misses      16569    16620      +51     
+ Partials     8889     8877      -12     

Files with missing lines	Coverage Δ
src/node.h	92.45% <ø> (ø)

... and 31 files with indirect coverage changes

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