doc: explain must dependences in the result of dataflow analysis
doc: explain must dependences in the result of dataflow analysis

Reported-by: Siddharth Bhat <3baaeb6f2f139c2f6426bc3b7155f0fdbf4a57f3@gmail.com>
Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

PollyLabs/isl,PollyLabs/isl,Meinersbur/isl,tobig/isl,inducer/isl-mirror,PollyLabs/isl,inducer/isl-mirror,Meinersbur/isl,tobig/isl,Meinersbur/isl,nicolasvasilache/isl,nicolasvasilache/isl,inducer/isl-mirror,inducer/isl-mirror,PollyLabs/isl,Meinersbur/isl,PollyLabs/isl,nicolasvasilache/isl,tobig/isl,tobig/isl,inducer/isl-mirror,nicolasvasilache/isl,nicolasvasilache/isl