[SYCL][Doc] Improve wording for GRF sizes

[gmlueck]

Improve the spec wording for the extension that allows applications to choose the GRF size of a kernel.
Also, move to our more modern specification style.

[YuriPlyakhin]

github has problems with comments per line, so I'll give feedback in general comment:

This extension adds the following properties that can be used when defining a kernel. However, using these properties to select a larger GRF size may impose a lower limit on the kernel’s maximum work-group size.

When I'm reading these statements, after the first sentence, and going to the second I'm expecting to see what the properties are, but they are introduced later. Maybe some description could be added to tell what those following properties are. Otherwise, second sentence is referring to them as "these properties", while they were not yet defined actually. And then to make sense out of it, I need to scan through document further and then return back.

[YuriPlyakhin]

Advanced users may want to increase this size if the kernel is known to need many registers. However, increasing this size may also reduce the maximum work-group size for the kernel.

and later

However, using these properties to select a larger GRF size may impose a lower limit on the kernel’s maximum work-group size.

Information is duplicated, is it possible to provide it in one place?

[gmlueck]

Thanks for the feedback, @YuriPlyakhin. I tried to address both comments in 101b364. Does this read better?