gh-157216: Clarify that set_forkserver_preload() does not cover lazy imports

[VimalN2005]

• Issue: Docs: set_forkserver_preload() does not cover lazily imported modules #157216

Description

Clarifies in Doc/library/multiprocessing.rst that set_forkserver_preload() only preloads modules imported at the top level.

Transitive dependencies that are imported lazily inside function or method bodies (for example, _strptime via datetime.datetime.strptime()) are not loaded in the forkserver process and will instead be imported separately in each child process upon first use.

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± library/multiprocessing.html

[mohsen-rahmati-dev]

Thanks for picking this up. I filed the issue and took the measurements, so two substantive notes:

1. "directly imported at top level" is wrong. Module-level imports are followed transitively — if a preloaded module imports A at module level and A imports B at module level, B is in the forkserver as well. Docs: set_forkserver_preload() does not cover lazily imported modules #157216 shows import importlib.metadata bringing in 75 modules that way, all inherited. The line is module-level vs. inside a function body, not depth. As written the paragraph makes preloading sound much weaker than it is.

2. The consequence is missing. "imported separately in each child process" doesn't tell the reader why to care: each child allocates that memory privately, so the cost scales with the number of children (~5.9 MiB per child for the two modules in the issue).

Also :meth:~datetime.datetime.strptime`` for the shortened form, and I'd put the paragraph right after the opening description rather than between the launch-order note and the on_error parameter.

I've opened docs-gh-157216 covering these. Fine by me whichever a reviewer picks.

[picnixz]

I will be picking the other PR as it ks the OP's PR.