Fix memory corruption in the aiGetExportFormatDescription() API. Clarify Exporter doc.

code/AssimpCExport.cpp

@@ -61,6 +61,8 @@ ASSIMP_API size_t aiGetExportFormatCount(void)
 // ------------------------------------------------------------------------------------------------
 ASSIMP_API const aiExportFormatDesc* aiGetExportFormatDescription( size_t pIndex)
 {
+	// Note: this is valid as the index always pertains to a builtin exporter,
+	// for which the returned structure is guaranteed to be of static storage duration.
 	return Exporter().GetExportFormatDescription(pIndex);
 }