docs: coding-style: Rewrite section on shortening comparisons

docs: coding-style: Rewrite section on shortening comparisons

The code style showed `bool hasFoos; if (hasFoos == true)` as a
good example in one place, only to warn against comparisons with
`true` a couple of paragraphs further down.

Merge this advice on comparing with `true` into the "Conditional
expressions" section and split the example up for readability.

Signed-off-by: Tim Wiederhake <twiederh@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>