Clarifications to backends section

[AerialMantis]

• Remove wording which repeats "see ..." text to make it flow better.

[gmlueck]

This does not read well in the rendered HTML because it looks like a sentence fragment. Can we just remove the "seealso" callout and write the sentence like:

For the name of the macro if the vendor defines the SYCL backend outside of the Khronos SYCL group, see section 6 of the core SYCL specification.

Where "section 6" is a link to that section?

This would read well, and I think it's a general improvement. Section 6 does not apply generally to the "Backend macros" section of the reference guide, so the "seealso" callout is kind of confusing. By writing it as I suggest, it's clear that section 6 applies only to that one sentence.