gh-157216: Document that forkserver preload skips deferred imports

[mohsen-rahmati-dev]

The current set_forkserver_preload() entry says the named modules' "already imported state is inherited by forked processes", but doesn't say what happens to a module that a preloaded module imports inside a function body rather than at module level. Those are not imported in the forkserver process, so each child imports them privately on first use.

This adds a paragraph after the opening description: module-level imports are inherited (transitively), imports inside function bodies are not, with datetime.strptime() / _strptime as the example, and a note that calling such a function at the module level of a preloaded module puts the import in the forkserver.

Measurements, module counts and a reproducer are in gh-157216.

Docs-only, so no Misc/NEWS.d entry.

[python-cla-bot]

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± library/multiprocessing.html