Objective-C API: Update public header documentation
https://bugs.webkit.org/show_bug.cgi?id=114841

Reviewed by Geoffrey Garen.

Added documentation for the newly added object lifetime-related stuff.

* API/JSManagedValue.h:
* API/JSVirtualMachine.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148974 268f45cc-cd09-0410-ab3c-d52691b4dbfc