CApiFFI is available since 7.6, not 7.10
GHC user's guide says that
CApiFFI language extension was added in 7.10: https://gitlab.haskell.org/ghc/ghc/-/blob/master/docs/users_guide/exts/ffi.rst#L390-393
This does not seem correct, because according to https://downloads.haskell.org/~ghc/7.6.3/docs/html/users_guide/ffi.html#ffi-capi
CApiFFI is available since 7.6.
Proposed improvements or changes
(BTW the typo leaked into https://www.haskell.org/ghc/blog/20210709-capi-usage.html as well)