doc: Clang 8 or later is required with FORCE_USE_SYSTEM_CLANG
Summary:
The usage of pragmas within the macOS SDK requires LLVM Clang 8. This is the version as our prebuilt Clang, however the minimum is worth noting here as they may diverge and/or expert users might expect they could use an earlier version.
Backport of core PR19617.
Depends on D7755.
Test Plan: Read the doc.
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D7756