Documentation/gpu: Document the situation with unqualified drm-memory-

Currently it is not well defined what is drm-memory- compared to other
categories.

In practice the only driver which emits these keys is amdgpu and in them
exposes the current resident buffer object memory (including shared).

To prevent any confusion, document that drm-memory- is deprecated and an
alias for drm-resident-memory-.

While at it also clarify that the reserved sub-string 'memory' refers to
the memory region component, and also clarify the intended semantics of
other memory categories.

v2:
 * Also mark drm-memory- as deprecated.
 * Add some more text describing memory categories. (Alex)

v3:
 * Semantics of the amdgpu drm-memory is actually as drm-resident.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.keonig@amd.com>
Cc: Rob Clark <robdclark@chromium.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>