bug#52453: Full timezone name

[積丹尼 Dan Jacobson]

On (info "(coreutils) Time conversion specifiers")

‘%Z’
     alphabetic time zone abbreviation (e.g., ‘EDT’), or nothing if no
     time zone is determinable.  See ‘%z’ for how it is determined.

Also mention something about how to get the full timezone name.

E.g., show instead of CST: Central Standard Time, or China Standard Time, or
China Central Standard Time, etc.