user guide: Document `ENABLE_RUNTIME_INSTRUMENTATION` option
user guide: Document `ENABLE_RUNTIME_INSTRUMENTATION` option

Change-Id: I8e50df67e860b9589834445761a7b9927690fdce
Signed-off-by: dp-arm <5172875e934a989df22d22726b6d9d921b91fbdd@arm.com>

lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware,sbranden/arm-trusted-firmware,sbranden/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware