Documentation: dma-buf: heaps: Add naming guidelines

Documentation: dma-buf: heaps: Add naming guidelines

We've discussed a number of times of how some heap names are bad, but
not really what makes a good heap name.

Let's document what we expect the heap names to look like.

Reviewed-by: Andrew Davis <afd@ti.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/all/20250728-dma-buf-heap-names-doc-v4-1-f73f71cf0dfd@kernel.org/
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>