Can any one help with my below query
'Where should design decisions that ultimately affect the end user(Maintainer) be recorded?"
Quick response would be design log. Slightly longer response is design log and record anything relevant on the design itself.
Taking a different viewpoint, all design decisions affect maintenance. However, "usual" decisions are not recorded and it is only exceptional/abnormal decisions that do require recording. It also depends on what, and why, you are trying to log some information. If it is to do with enabling a maintainer to maintain their equipment more efficiently then it may be better to log it and include it in any training given. If it is a safety issue then standards (in mainline UK) state very clearly what you need to do under CDM. It also details who is the designer (not always a designer) and who is responsible for recording the information and where!
1) technical decision - design log (and potentially on design).
2) engineering or safety decision - engineers' hazard log.
3) contractual/commercial/safety - project hazard log.