par-tile opt-option needs to be documented

[FabioLuporini]

copy-pasting from slack:

We need to document the par-tile opt option. AFAICT, it is only showcased here https://github.com/devitocodes/devito/blob/master/tests/test_gpu_openacc.py#L150 . We need a paragraph here: https://github.com/devitocodes/devito/blob/master/examples/performance/00_overview.ipynb also, par-tile usable on CPUs to hard code the block shape (should it ever be needed — the compiler uses the feature for other purporses)