PEP 831: Clarify build backend flag propagation #5073

PEP 831 currently overstates how CPython build flags propagate through generic build backends. Update the PEP to distinguish Setuptools and deliberate sysconfig consumers from other PEP 517 backends, and document build-details.json as the appropriate future standardized interface.

Context: https://discuss.python.org/t/pep-831-frame-pointers-everywhere-enabling-system-level-observability-for-python/106958/7