[3.14] gh-135805: Document the X option and env var for controlling thread-local bytecode (GH-135868) (#135897)

gh-135805: Document the X option and env var for controlling thread-local bytecode (GH-135868)

Document the X option and env var for controlling thread-local bytecode.
(cherry picked from commit fea5ccc55d8486300beb1d0254da030a4da10394)

Co-authored-by: mpage <mpage@meta.com>