emulators/rpcs3: document Mesa quirks

On Intel Skylake GT2 running Persona 5 with Mesa 19.0.8 either crashes (Vulkan)
or has missing graphics (OpenGL).

RPCS3: Assertion Failed! Vulkan API call failed with unrecoverable error: Requested feature not available (VK_ERROR_FEATURE_NOT_PRESENT)
(in file rpcs3/Emu/RSX/VK/VKHelpers.h:913)