Add note about clear messages for more that 500 entries

[ogail]

check Remarks section: http://msdn.microsoft.com/en-us/library/windowsazure/dd179454.aspx

[jcookems]

To clarify, the remarks say that if there are a large number of messages, the clear might only remove a subset of the messages and return a 500 error code. In that situation, it is up to the user code to retry the clear call until success is returned, indicating that all messages are finally removed.

[guangyang]

This is too old without any customer feedback. Close it.