gh-122826: Document exec closure name resolution

[kn1g78]

Fixes #122826.

The execution model documentation previously described name resolution for eval() and exec() without distinguishing string source from code objects. This change limits the existing explanation to string source and documents that exec() can explicitly receive closure cells for a code object containing free variables.

Checks:

• Doc\make.bat check
• Doc\make.bat html reference/executionmodel.rst with warnings treated as errors

• Issue: Execution model reference should cover closure arg to exec & eval #122826

[read-the-docs-community]

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± reference/executionmodel.html