Add withinSubject usage

To go along with cypress-io/cypress#2791

lib/tags/usage.js

@@ -33,6 +33,10 @@ module.exports = function usageOptions (hexo, args) {
     return 'Serially click multiple elements'
   }
 
+  const withinSubject = () => {
+    return 'Element to search for children in. If null, search begins from root-level DOM element'
+  }
+
   switch (opt) {
     case 'log':
       return log()
@@ -42,6 +46,8 @@ module.exports = function usageOptions (hexo, args) {
       return multiple()
     case 'timeout':
       return timeout()
+    case 'withinSubject':
+      return withinSubject()
     default:
       // error when an invalid usage option was provided
       throw new Error(`{% usage_options %} tag helper was provided an invalid option: ${opt}`)

source/api/commands/get.md

@@ -51,6 +51,7 @@ Pass in an options object to change the default behavior of `cy.get()`.
 | --------- | -------------------------------------------------------- | ---------------------------------- |
 | `log`     | `true`                                                   | {% usage_options log %}            |
 | `timeout` | {% url `defaultCommandTimeout` configuration#Timeouts %} | {% usage_options timeout cy.get %} |
+| 'withinSubject' | null                                               | {% usage_options withinSubject %} |
 
 ## Yields {% helper_icon yields %}