Move docstring of `seq` to primops.txt.pp

The documentation for `seq` was recently augmented via #9390 &
cbfa107604f4cbfaf02bd633c1faa6ecb90c6dd7. However, it doesn't show
up in the Haddock generated docs because `#ifdef __HADDOCK__` doesn't
work as expected.  Also, it's easier to just fix the problem at the
origin (which in this is case is the primops.txt.pp file).

The benefit/downside of this is that now the extended documentation
shows up everywhere `seq` is re-exported directly.
Detabify primops.txt.pp