gh-134537: Clarify context manager reuse in ContextDecorator - #159848

Correct the ContextDecorator equivalence example to reuse one context manager instance across calls. The previous example constructed a new instance inside the function, which can behave differently for stateful managers.

Clarify the requirements for recursive and concurrent calls, and distinguish the per-call recreation provided by contextmanager().

Fixes #134537.

Validation:

• Verified shared instance identity across sequential and recursive decorated calls, and distinct contexts for a generator-based decorator, on the current free-threaded debug build.
• Full HTML documentation build with warnings treated as errors.
• Sphinx lint and patchcheck.

• Issue: ContextDecorator documentation is unclear. #134537