add documentation for aria-modal prop (facebook#3320)

Author: dakshbhardwaj

docs/accessibility.md

@@ -204,6 +204,16 @@ A Boolean value indicating whether the accessibility elements contained within t
 
 For example, in a window that contains sibling views `A` and `B`, setting `accessibilityElementsHidden` to `true` on view `B` causes VoiceOver to ignore the elements in the view `B`. This is similar to the Android property `importantForAccessibility="no-hide-descendants"`.
 
+### `aria-modal` <div class="label ios">iOS</div>
+
+Boolean value indicating whether VoiceOver should ignore the elements within views that are siblings of the receiver.
+
+| Type | Default |
+| ---- | ------- |
+| bool | false   |
+
+---
+
 ### `importantForAccessibility` <div class="label android">Android</div>
 
 In the case of two overlapping UI components with the same parent, default accessibility focus can have unpredictable behavior. The `importantForAccessibility` property will resolve this by controlling if a view fires accessibility events and if it is reported to accessibility services. It can be set to `auto`, `yes`, `no` and `no-hide-descendants` (the last value will force accessibility services to ignore the component and all of its children).

docs/view.md

@@ -249,6 +249,16 @@ When `true`, indicates that the view is an accessibility element. By default, al
 
 ---
 
+### `aria-modal` <div class="label ios">iOS</div>
+
+Boolean value indicating whether VoiceOver should ignore the elements within views that are siblings of the receiver. Has precedence over the `accessibilityViewIsModal` prop.
+
+| Type | Default |
+| ---- | ------- |
+| bool | false   |
+
+---
+
 ### `collapsable` <div class="label android">Android</div>
 
 Views that are only used to layout their children or otherwise don't draw anything may be automatically removed from the native hierarchy as an optimization. Set this property to `false` to disable this optimization and ensure that this `View` exists in the native view hierarchy.