primops: make nature of foldl' strictness clearer

* Clarify the documentation of foldl': That the arguments are forced
  before application (?) of `op` is necessarily true. What is important
  to stress is that we force every application of `op`, even when the
  value turns out to be unused.

* Move the example before the comment about strictness to make it less
  confusing: It is a general example and doesn't really showcase anything
  about foldl' strictness.

* Add test cases which nail down aspects of foldl' strictness:
  * The initial accumulator value is not forced unconditionally.
  * Applications of op are forced.
  * The list elements are not forced unconditionally.

tests/lang/eval-okay-foldlStrict-lazy-elements.nix

@@ -0,0 +1,9 @@
+# Tests that the rhs argument of op is not forced unconditionally
+let
+  lst = builtins.foldl'
+    (acc: x: acc ++ [ x ])
+    [ ]
+    [ 42 (throw "this shouldn't be evaluated") ];
+in
+
+builtins.head lst