Added info on bookmark return values for securityScopedBookmarks

Editor assessment

The subject states the addition: return-value documentation for securityScopedBookmarks, with the option named exactly. The docs type is accurate, and the absent scope is tolerable. No body is present, which is acceptable for a focused documentation addition whose target and content the subject names, though the bookmark format itself would have been worth a snippet. The PR reference is well-formed. Imperative mood, no trailing period, slightly long subject. A precise, self-sufficient docs message for a change of this size.