Returns a localised string describing a source element handling the protocol specified in
protocol, for use in error dialogs or
other messages to be seen by the user.
This function is mainly for internal use, applications would typically use missing_plugin_message_get_description to get a description of a missing feature from a missing-plugin message.
the protocol the source element needs to handle, e.g. "http"
a newly-allocated description string. Free string with g_free when not needed any longer.