Adds syntax tests, documentation and changelog entry.

Refines comment for array utility function.

test/libsolidity/syntaxTests/array/array_pop_arg.sol

@@ -0,0 +1,8 @@
+contract C {
+    uint[] data;
+    function test() public {
+      data.pop(5);
+    }
+}
+// ----
+// TypeError: (65-76): Wrong argument count for function call: 1 arguments given but expected 0.