Help: Document CMAKE_OSX_SYSROOT default change in 4.0

In commit 7b19531291 (macOS: Do not pass any SDK/-isysroot to compilers
by default, 2024-11-06, v4.0.0-rc1~511^2) we forgot to update the
documentation of `CMAKE_OSX_SYSROOT`.

Fixes: #26846
2 files changed