docs: add non-return function and if shorthand tips

docs: add non-return function and if shorthand tips

Non-return functions should not be combined with `else' clause.

The if shorthands `var = e ? t : f;' need to fit to single line,
and if that does not look good use normal "if else" syntax.

Both tips are mentioned in email bellow.

http://www.spinics.net/lists/util-linux-ng/msg05152.html

Signed-off-by: Sami Kerola <kerolasa@iki.fi>