Add some documentation

Add some documentation

Swap if ( $foo ) { $this->addFields( 'foo' ); } for $this->addFieldsIf( 'foo', $foo ); etc

Also swap $this->addFieldsIf( 'foo', $foo ); $this->addFieldsIf( 'foo2', $foo ); for $this->addFieldsIf( array( 'foo', 'foo2' ), $foo );

Micro-optimisation and readability!