Update dr_module_preferred_name() documentation

_From [m...@google.com](https://code.google.com/u/109256567295486965295/) on October 06, 2014 11:26:35_

Please update documentation for dr_module_preferred_name(), it doesn't say whether we are supposed to free or not the returned pointer.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1548_

Find the API documentation for dr_module_preferred_name(); begin by checking how its returned pointer is described. Update the ownership guidance to state whether callers must free it, then review the surrounding documentation for consistency.

Written by the indexing model from the issue text.