#111127 follow up patchby myself: document the node_load internal cache and external cache, and how they interoperate

modules/node/node.module

@@ -601,6 +601,17 @@ function node_invoke_nodeapi(&$node, $op, $a3 = NULL, $a4 = NULL) {
 /**
  * Load a node object from the database.
  *
+ * This function provides two levels of caching. The internal cache stores
+ * all nodes loaded in this request. Use the $reset argument to invalidate
+ * it. The cache API based cache stores all nodes throughout requests,
+ * until invalidated with cache_clear_all().
+ *
+ * If the load operation was cached previously, we load the data from
+ * that cache, and no nodeapi load operations are called. That means,
+ * that we expect all load operations to return the same data and do
+ * not use any conditions on the user, language or anything else,
+ * which would limit what is stored in the node_load cache.
+ *
  * @param $param
  *   Either the nid of the node or an array of conditions to match against in the database query
  * @param $revision