docs/design: Add missing sections for Isolation APIs

Add sections for the following missing functions within the
Isolation API documentation (hardware_abstraction_layer):
- tfm_hal_verify_static_boundaries
- tfm_hal_boundary_need_switch

Also, tidy indentation and provide missing 'Parameters' description
for some functions.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I026b90cbc95db448bae19857e3ad1bbf27f0c3de
1 file changed