doc: review interrupts_handling
doc: review interrupts_handling

interrupts_handling.md covers interrupts (IRQ/FIQ) and SMC exceptions.
Clean title and state it from the introduction the document coverage.

Review sections split to clarify content.
Detail main routing of IRQ, FIQ and SMC events.
Clarify Monitor in ARMv7/ARMv8 cases.

Use 'trusted thread' to refer the threads management in OP-TEE core.

Add a section about OP-TEE thread scheduling.

Signed-off-by: Etienne Carriere <f21bc5b8f3ea1962eb45af359f1abea6578cab94@linaro.org>
Reviewed-by: Igor Opaniuk <ca782e5848fbe7ab0a250faa8ae33b643dd89a09@linaro.org>
Reviewed-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os