Clarify svm_ptr argument to clEnqueueSVMMemFill

Found while discussing and developing the unified SVM extension: the current description for the _svm_ptr_ argument to [clEnqueueSVMMemFill](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#clEnqueueSVMMemFill) is confusing and should be improved. For reference, the current text is:

> svm_ptr is a pointer to a memory region that will be filled with pattern. It must be aligned to pattern_size bytes. If svm_ptr is allocated using [clSVMAlloc](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#clSVMAlloc) then it must be allocated from the same context from which command_queue was created. Otherwise the behavior is undefined.

There are three specific _svm_ptr_ cases I believe we need to document:

1. _svm_ptr_ is unaligned. This is a `CL_INVALID_VALUE` error.
2. _svm_ptr_ is `NULL`. This is a `CL_INVALID_VALUE` error.
3. _svm_ptr_ was allocated using clSVMAlloc from a different context. This is undefined behavior (note: even if the device supports system SVM?).
4. _svm_ptr_ was not allocated using clSVMAlloc. This is valid if the device associated with the _command_queue_ supports system SVM, otherwise this is undefined behaivor.
5. _svm_ptr_ was allocated using clSVMAlloc from the same context. This is a valid.

Debatably cases (3) and (4) should be errors instead of undefined behavior, but this would be a change compared to the current spec. Note though:

* It is possible for an implementation to return an error in these cases (because behavior is undefined), but this would not be required.
* If we _really_ wanted to, we could consider tightening the behavior for implementations supporting unified SVM.

Start at the clEnqueueSVMMemFill entry in the OpenCL API specification and compare the current svm_ptr wording with the five cases listed in the issue. Clarify the alignment, NULL, context, system-SVM, and clSVMAlloc requirements, while checking the surrounding error and undefined-behavior language; done means the specification explicitly covers each case.

Written by the indexing model from the issue text.