Clarify the docs for when write and end can be used.

lib/index.js

@@ -820,6 +820,9 @@ Stream.prototype.resume = function () {
  * any [Node Readable Streams](http://nodejs.org/api/stream.html#stream_class_stream_readable)
  * you pipe in.
  *
+ * Only call this function on streams that were constructed with no source
+ * (i.e., with `_()`).
+ *
  * @id end
  * @section Stream Objects
  * @name Stream.end()
@@ -1177,6 +1180,9 @@ Stream.prototype.pull = function (f) {
  * You shouldn't need to call this yourself, but it may be called by Node
  * functions which treat Highland Streams as a [Node Writable Stream](http://nodejs.org/api/stream.html#stream_class_stream_writable).
  *
+ * Only call this function on streams that were constructed with no source
+ * (i.e., with `_()`).
+
  * @id write
  * @section Stream Objects
  * @name Stream.write(x)
@@ -1191,6 +1197,10 @@ Stream.prototype.pull = function (f) {
  * xs.toArray(function (ys) {
  *     // ys will be [1, 2]
  * });
+ *
+ * // Do *not* do this.
+ * var xs2 = _().toArray(_.log);
+ * xs2.write(1); // This call is illegal.
  */
 
 Stream.prototype.write = function (x) {