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

@@ -3180,9 +3180,14 @@ static RegisterPrimOp primop_foldlStrict({
     .doc = R"(
       Reduce a list by applying a binary operator, from left to right,
       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.
+      ...`.
+      For example, `foldl' (acc: elem: acc + elem) 0 [1 2 3]` evaluates
+      to `6` and `foldl' (acc: elem: { "${elem}" = elem; } // acc) {}
+      ["a" "b"]` evaluates to `{ a = "a"; b = "b"; }`.
+      The first argument of `op` is the accumulator wheres the second
+      argument is the current element being processed. The return value
+      of each application of `op` is evaluated immediately, even for
+      intermediate values.
     )",
     .fun = prim_foldlStrict,
 });