[SYCL][Doc] Clarify that uniform is immutable

[Pennycook]

We received feedback from the oneAPI TAB that it was not immediately
obvious that the uniform class is immutable. The changes here are an
attempt to clarify this:

• The implicit conversion operator returns "const T", reflecting that
  the user is not getting a reference to the underlying data.

• The assignment operator is deleted, preventing the user from
  accidentally assigning one uniform object to another.

• Other operators that users might accidentally call on a uniform
  of a fundamental type (e.g. += on a uniform) are explicitly
  deleted; the intent here is to give a simplified error message (rather
  than a complex C++ diagnostic) that signals the operator is
  deliberately not supported (rather than accidentally missing or
  unimplemented).

Signed-off-by: John Pennycook [email protected]

[Pennycook]

@rolandschulz, @intel/dpcpp-specification-reviewers: Ping

[Pennycook]

Thanks, @rolandschulz. @bader, I think this can be merged now.