gh-95245: Document Py_TPFLAGS_PREHEADER

[sergey-miryanov]

I see following usages of Py_TPFLAGS_PREHEADER:

1. Calculate size of preheader (but preheader also contains GC info)
2. Check if types are support managed dict or managed weakref

I checked all linked PR from #95245 and can't find any explanation. I also checked https://github.com/python/cpython/blob/main/Objects/object_layout.md.

@encukou Please take a look.

• Issue: Reduce the size of plain Python objects from 8 to 6 machine words. #95245

[ZeroIntensity]

We should include documentation on the type object page too.

[sergey-miryanov]

@ZeroIntensity Thanks for review! Fixed (I hope :)

[ZeroIntensity]

I think that if this is internal-only, it would be better to completely drop it from public headers rather than document it as such.

[ZeroIntensity]

We don't need news entries for documentation changes, I'll put skip news on this.

[sergey-miryanov]

Thanks! I will remove news entry as we got consensus.

[ZeroIntensity]

I think that if this is internal-only, it would be better to completely drop it from public headers rather than document it as such.

(Don't do this yet, by the way. Let's come to a consensus in the issue.)