Mixed

"Needs to be registered"
"and the GPU supports it, to use it"
"If the users want"

Also, the description of how to use the Theano flag is a bit hard to parse. Maybe something along the line of : 
"If the users want to use the legacy convolution, they need to use the Theano flag ``optimizer_excluding``. If cudnn is not available, only the optimization local_conv_gemm needs to be disabled : 
[PUT FLAG DEFINITION HERE]
If cudnn is available, the optimization local_gpu_conv also needs to be disabled : 
[DEFINE FLAGS TO EXCLUTE BOTH OPTIMIZATIONS]"