anv: document cache flushes & invalidations

anv: document cache flushes & invalidations

A little bit of explanation regarding how vkCmdPipelineBarrier()
works.

v2: Avoid referring to data port cache when it's actually sampler
    caches (Jason)
    Complete explanation for indirect draws (Jason)

v3: s/samplers/sampler/ (Jason)
    s/UBOs/data port/
    Add documentation for VK_ACCESS_CONDITIONAL_RENDERING_READ_BIT_EXT (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com> (v1)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net> (v2)