Bug 1755481: correct documentation of `nsIClipboard::getData`. r=mccr8

Before, it was confusing. One could have assumed that the method gets
the data for all flavors.

Differential Revision: https://phabricator.services.mozilla.com/D138776

widget/nsIClipboard.idl

@@ -33,11 +33,13 @@ interface nsIClipboard : nsISupports
                     in long aWhichClipboard ) ;
 
    /**
-    * Given a transferable, get the clipboard data.
+    * Filters the flavors aTransferable can import (see
+    * `nsITransferable::flavorsTransferableCanImport`) and gets the data for the
+    * first flavor. That data is set for aTransferable.
     *
     * @param  aTransferable The transferable
     * @param  aWhichClipboard Specifies the clipboard to which this operation applies.
-    * @result NS_Ok if no errors
+    * @result NS_OK if no errors
     */
 
     void getData ( in nsITransferable aTransferable, in long aWhichClipboard ) ;