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.

1 files changed, 11 insertions, 2 deletions

diff --git a/compiler/prelude/primops.txt.pp b/compiler/prelude/primops.txt.pp
index d5566fe363..b1a42b3d1f 100644
--- a/compiler/prelude/primops.txt.pp
+++ b/compiler/prelude/primops.txt.pp
@@ -2551,8 +2551,17 @@ pseudoop "proxy#"
 
 pseudoop   "seq"
    a -> b -> b
-   { Evaluates its first argument to head normal form, and then returns its second
-	argument as the result. }
+   { The value of {\tt seq a b} is bottom if {\tt a} is bottom, and
+     otherwise equal to {\tt b}. {\tt seq} is usually introduced to
+     improve performance by avoiding unneeded laziness.
+
+     A note on evaluation order: the expression {\tt seq a b} does
+     {\it not} guarantee that {\tt a} will be evaluated before {\tt b}.
+     The only guarantee given by {\tt seq} is that the both {\tt a}
+     and {\tt b} will be evaluated before {\tt seq} returns a value.
+     In particular, this means that {\tt b} may be evaluated before
+     {\tt a}. If you need to guarantee a specific order of evaluation,
+     you must use the function {\tt pseq} from the "parallel" package. }
 
 primtype Any
 	{ The type constructor {\tt Any} is type to which you can unsafely coerce any