vkd3d-shader/hlsl: Add field-level documentation to the most important structs in hlsl.h.

I believe this information could be very useful for newcomers to the HLSL compiler.

Documentation often needs to reference other parts of the code, so it can be a problem to update these references if the referenced code is modified. However, I think that documentation in the data definitions (as opposed to documentation inside the functions) is easier to maintain, since each data type is declared in a single place, to which one often keeps going back for reference, so it is less probable to forget to update a comment.

Also, it opens up the possibility of explaining what things are, instead of how they are used, in some cases, avoiding these references to other places in the code.

I can think on the following additional benefits:

• Updates in these documentation comments, when the way a field is used changes, or when new fields are added, can help making future commits more readable.
• Some IDEs display the comments above the field/struct declaration when the cursor is over an use of this field/struct.

A hopefully not deceiving sketch: