doc: drop recommendation against bool in structures

The coding style advised against bool in structures, citing space and
type-size concerns inherited from the Linux kernel. That does not match
DPDK practice: bool is already used in over 1400 structure fields across
the tree, including core APIs such as rte_flow. Drop the note so the
style guide reflects actual usage.

Signed-off-by: Maxime Leroy <maxime@leroys.fr>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Marat Khalili <marat.khalili@huawei.com>