Add missing '@throws PostmarkExcpetion', fix processRestRequest() @return type

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Overview A small number of docblock fixes to better support IDEs

Changes

• add missing @return PostmarkException tag to all relevant methods
• fix PostmarkClientBase::processRestRequest() @return type (was object, should be array)

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@atheken No rush in merging this, just adds a few nice docblock changes to keep IDEs a bit happier