Add documentation for GraphicsPipelineAbstract

[rukai]

• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes - in this repository
• Updated documentation to reflect any user-facing changes - PR to the guide that fixes existing documentation invalidated by this PR.

This PR better documents GraphicsPipelineAbstract by mentioning that your draw calls will now need the buffer to be wrapped in a vec.

I'm not convinced this is the best solution though, the ideal solution would be for GraphicsPipeline and GraphicsPipelineAbstract to function in the same way here. That is, both should allow a CpuAccessibleBuffer to be passed directly in the draw call.

I have no idea how to implement that though, the generics is doing my head in.
The code that handles it for GraphicsPipeline is here:

vulkano/vulkano/src/pipeline/graphics_pipeline/mod.rs

Line 504 in 7a11120

unsafe impl<Mv, L, Rp, S> VertexSource<S> for GraphicsPipeline<Mv, L, Rp>

We need to find a way to implement this for GraphicsPipelineAbstract.
If such a thing is impossible then I will merge this PR instead.

The commit to add the linked code is 4c0a093
The commit to add VertexSource<Vec<Arc<BufferAccess + Send + Sync>>> is 64459fc#diff-1f6ce30b101af55aab9d8ea160fee761

As mentioned in the issue, it looks like this will be resolved by later rust features, so I'll merge this as is and keep the issue open. #1082