man: explain why TZ=: is used

man: explain why TZ=: is used

Also, reword the description a bit. "As a string" is meaningless in the context
of commandline arguments, where evyrything is a string. This is not a
strongly-typed programming language where 5 is a number but "5" is something
completely different. Here both 5 and "5" are indistinguishable. The original
text was trying to say that a location name should be given and not a number,
so say "time zone location name".

For #17177.