Add a new FAQ section explaining how to ensure parenthesis

Add a new FAQ section explaining how to ensure parenthesis
with op().

Not sure if this can be further improved at some point, such
as if a BinaryExpression automatically applied self_group()
when op() is called, however for the moment op() behaves consistently
as with all other operators without any ad-hoc effects.

Change-Id: Ia7f1ab43990450bd96757848b77a35e8fedeab63
References: #4174