doc: primops: add more info for foldl

From the existing doc it is not obvious whether the first or the
second argument is the accumulator. This is however relevant to
know, as for certain scenarios, this might change the behavior.

src/libexpr/primops.cc

@@ -3182,7 +3182,9 @@ static RegisterPrimOp primop_foldlStrict({
       e.g. `foldl' op nul [x0 x1 x2 ...] = op (op (op nul x0) x1) x2)
       ...`. For example, `foldl' (x: y: x + y) 0 [1 2 3]` evaluates to 6.
       The return value of each application of `op` is evaluated immediately,
-      even for intermediate values.
+      even for intermediate values. The first argument of the operation
+      (`x` in the example above) is the accumulator wheres the second one
+      (`y`) is the next element.
     )",
     .fun = prim_foldlStrict,
 });