Update cross origin explainer (#908)

updating the description of the install parameter based on #894

also, removing the part about adding to manifest's install_source as it no longer applies.

WebInstall/explainer_cross_domain.md

@@ -107,9 +107,9 @@ If the `manifest_id` is the *what* to install, the `install_url` is the *where*
 
 Unless the UA decides to [gate this functionality behind installation](#gating-capability-behind-installation), the behaviour between calling the `install` method on a tab or on an installed application should not differ. The install method can be used in two different ways.
 
-1. `navigator.install(manifest_id, <install_url> [, <params>])`: This signature of the method requires the id of the application to be installed (`manifest_id`), and the installation location for the app (`install_url`). This is the most common API use case the API for cross-origin scenarios.
+1. `navigator.install(manifest_id, <install_url> [, <params>])`: The method takes an id and install url and tries to install the web content at `install_url`. Installation will proceed if the following are true: a. The target web app links to a manifest. b. The target web app's manifest includes either an `id` or `start_url`. c. The `id` parameter matches the declared or resolved id of the target web app. This is the most common API use case the API for cross-origin scenarios.
 
-This will prompt for installation of the app if the requesting origin has installation permissions (see [security section](#integration-with-the-permissions-api)) and the target application has specified this domain in its `install_sources` manifest field.
+This will prompt for installation of the app if the requesting origin has installation permissions (see [security section](#integration-with-the-permissions-api)).
 
 #### **Parameters**