Document differences between displayed time zone and time zone reported by programs

Once we have #6284 we should document better the different between the time zone displayed on the desktop (localized) and the one reported by programs (like Pidgin and Icedove).

Parent Task: #6284

Related issues

Related to #12094
Related to #10819 (closed)

Original created by @sajolida on 10434 (Redmine)

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