docs(minerr): fill in error message descriptions

Errors I've documented so far:
- `$injector:cdep`
- `$injector:itkn`
- `$injector:modulerr`
- `$injector:nomod`
- `$injector:pget`
- `$injector:unpr`
- `ng:areq`
- `ng:cpi`
- `ng:cpws`
- `ngModel:noass`

Closes #3430

docs/content/error/injector/unpr.ngdoc

@@ -2,3 +2,25 @@
 @name $injector:unpr
 @fullName Unknown Provider
 @description
+
+This error results from the `$injector` being unable to resolve a required
+dependency. To fix this, make sure the dependency is defined and spelled
+correctly. For example:
+
+```
+angular.module('myApp', [])
+  .controller('myCtrl', ['myService', function (myService) {
+    // Do something with myService
+  }]);
+```
+
+This code will fail with `$injector:unpr` if `myService` is not defined. Making
+sure each dependency is defined will fix the problem.
+
+```
+angular.module('myApp', [])
+  .service('myService', function () { /* ... */ })
+  .controller('myCtrl', ['myService', function (myService) {
+    // Do something with myService
+  }]);
+```