[oneDPL] Improve clarity in buffer wrapper usage

[danhoeflinger]

Improve clarity and consistency in how we describe buffer position objects, (the return of oneapi::dpl::begin/end), and how they interact with oneDPL iterators and the indirectly device accessible trait.

This does not change anything about the semantics of the specification, but hopes to improve clarity and add some text which was previously implied.

[Copilot]

Pull Request Overview

This PR improves clarity and consistency in the oneDPL documentation by better describing buffer position objects and their relationship with oneDPL iterators and the indirectly device accessible trait.

Key changes:

• Enhanced documentation of buffer position objects and their behavior with device policies
• Added clarification on how oneDPL iterators interact with buffer position objects
• Improved terminology consistency by replacing "unspecified iterator-like type" with proper references to buffer position objects

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
source/elements/oneDPL/source/parallel_api/iterators.rst	Enhanced documentation of buffer position objects, indirectly device accessible iterators, and improved terminology consistency
source/elements/oneDPL/source/parallel_api/execution_policies.rst	Updated description to clarify that SYCL buffers are passed as buffer position objects

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