Fix link + docs (#2476)

* Fix a copy/paste fail

Also, fix some incorrect tests.

* Clarify how routes are diffed, improve key + route resolver docs

- Add some missing links to route resolvers and single-child keyed
  fragments, clarify usage around them.
- Drive-by: remove a redundant sentence that itself was missing a
  period.

* Actually test for propagation and preventDefault

Previously, the mocks were both junk and inaccurate. No wonder my tests
were silently failing - they were wrong and not obviously wrong.

docs/keys.md

@@ -2,6 +2,7 @@
 
 - [What are keys](#what-are-keys)
 - [How to use](#how-to-use)
+	- [Single-child keyed fragments](#single-child-keyed-fragments)
 - [Debugging key related issues](#debugging-key-related-issues)
 
 ---
@@ -75,7 +76,9 @@ function correctUserList(users) {
 }
 ```
 
-Also, you might want to reinitialize a component. You can use the common pattern of a single-child keyed fragment where you change the key to destroy and reinitialize the element.
+#### Single-child keyed fragments
+
+Sometimes, you might want to reinitialize a component on command. You can use the common pattern of a single-child keyed fragment where you change the key to destroy and reinitialize the element.
 
 ```javascript
 function ResettableToggle() {
@@ -96,6 +99,20 @@ function ResettableToggle() {
 }
 ```
 
+You can also bind it to a known identity, for things like item views where you need to fetch a remote resource based on an ID. It's usually simpler than implementing all the logic to diff the ID and re-fetch a resource if it changes.
+
+```javascript
+function Page() {
+	return {
+		view: function() {
+			return m(Layout, [
+				[m(ItemView, {key: m.route.param("id")})],
+			])
+		}
+	}
+}
+```
+
 ---
 
 ### Debugging key related issues
@@ -120,7 +137,7 @@ users.map(function(u) {
 If you refactor the code and make a user component, the key must be moved out of the component and put on the component itself, since it is now the immediate child of the array.
 
 ```javascript
-// AVOID
+// AVOID - doesn't work
 var User = {
 	view: function(vnode) {
 		return m("div", { key: vnode.attrs.user.id }, [
@@ -137,7 +154,7 @@ users.map(function(u) {
 
 #### Avoid wrapping keyed elements in arrays
 
-Arrays are [vnodes](vnodes.md), and therefore keyable. You should not wrap arrays around keyed elements
+Arrays are [vnodes](vnodes.md), and therefore keyable. You should not wrap arrays around keyed elements outside [single-child keyed fragments](#single-child-keyed-fragments).
 
 ```javascript
 // AVOID