Adding aria-describedby attribute the draft editor

Summary:
We have a ariaDescribedBy prop in DraftEditorProps.js but it is not set in DraftEditor.
Currently the aria-describedby attribute is set based on if there is a placeholder and is set to the placeholderId.
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_aria-describedby_attribute

issue on github which also has suggested the fix: #1214

Reviewed By: jessebeach, flarnie

Differential Revision: D6154932

fbshipit-source-id: d828ca5f73921c5d303820c5aed5b39eda08ccd1

src/component/base/DraftEditor.react.js

@@ -264,7 +264,7 @@ class DraftEditor extends React.Component<DraftEditorProps, State> {
             aria-autocomplete={readOnly ? null : this.props.ariaAutoComplete}
             aria-controls={readOnly ? null : this.props.ariaControls}
             aria-describedby={
-              this._showPlaceholder() ? this._placeholderAccessibilityID : null
+              this.props.ariaDescribedBy || this._placeholderAccessibilityID
             }
             aria-expanded={readOnly ? null : ariaExpanded}
             aria-label={this.props.ariaLabel}