Bug: devguide gives incorrect advice for make -j

[Yhg1s]

Describe the bug

The devguide suggests using make -j to build CPython in a few places, which is not actually a good idea. make -j means no limit on the number of parallel jobs, not "a sensible limit". It already uses make -j4 in a few places, and that's a very reasonable default that will work well for machines with 2-10 cores. Let's do that everywhere.

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Additional context

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[Yhg1s]

Fixed by #1541.