slice: Add documentation.

doc/API.md

@@ -22,7 +22,7 @@
     - [push(value) -> async bufferLength](#pushvalue---async-bufferlength)
     - [reduce(callbackfn[, initialValue])](#reducecallbackfn-initialvalue)
     - [shift() -> async](#shift---async)
-    - [slice(start, end) -> Channel](#slicestart-end---channel)
+    - [slice(start[, end]) -> Channel](#slicestart-end---channel)
 - [Functional API](#functional-api)
 
 <!-- /TOC -->
@@ -278,7 +278,13 @@ Closed channels always return `undefined` immediately.
 The shift can be cancelled before completion by calling `cancel` on the
 returned promise.
 
-### slice(start, end) -> Channel
+### slice(start[, end]) -> Channel
+
+Return a new channel generated by skipping `start` number of values.  If `end`
+is provided then close the new channel after `end` - `start` values have been
+pushed.
+
+Unlike in `Array`'s method, `start` and `end` cannot be negative.
 
 # Functional API