man: replace STDOUT with standard output, etc.

man: replace STDOUT with standard output, etc.

Actually 'STDOUT' is something that doesn't appear anywhere: in the
stdlib we have 'stdin', and there's only the constant STDOUT_FILENO,
so there's no reason to use capitals. When refering to code,
STDOUT/STDOUT/STDERR are replaced with stdin/stdout/stderr, and in
other places they are replaced with normal phrases like standard
output, etc.