Documentation todos

[JMicheli]

This PR addresses a number of items (mostly) related to documentation/commenting and marked "TODO" in the repository.

Specifically it:

• Shortens the syntax for specifying image attachment usage in examples/src/bin/deferred/frame/system.rs as noted in a TODO.
  • This required adding a new function to ImageUseage, transient_input_attachment(), which creates an ImageUseage with the appropriate settings for that use. This is likely to be useful in other instances as well (see vulkano/src/image/usage.rs).
• Adds a small paragraph describing push constants to examples/src/bin/push-constants.rs (note that the language used was borrowed and adapted from the Vulkan specification for accuracy).
• Removes a TODO that was directed at users instead of maintainers in examples/src/bin/texture_array/main.rs.
• Adds a detailed explanation of the shader! macro to examples/src/bin/triangle-v1_3.rs and examples/src/bin/triangle.rs.
• Adds an actual example of generating a command buffer that would do something to the documentation in vulkano/src/command_buffer/mod.rs.
• Adds checks for max_framebuffer_height, max_framebuffer_width, and max_framebuffer_layers compliance to the constructor for ImageAttachment (see vulkano/src/image/attachment.rs).

All tests pass on my machine. This is my first ever pull request to a team project, so please forgive any errors I may have made. I am happy to make changes as needed to get this into shape for inclusion in the master branch.