[oneDPL] Create a subsection for is_execution_policy type trait

[akukanov]

This PR describes oneDPL is_execution_policy trait in more detail, and also addresses #558.

[danhoeflinger]

Generally LGTM, optional comment about the decay note.

[danhoeflinger]

LGTM

[rarutyun]

Overall, looks good to me. I left some small comments that do look as improvements to me. But you might disagree

[rarutyun]

Suggested change

	constexpr bool is_execution_policy_v = oneapi::dpl::is_execution_policy<T>::value;
	inline constexpr bool is_execution_policy_v = oneapi::dpl::is_execution_policy<T>::value;

[akukanov]

It's a template, inline should not be needed. See the standard (https://eel.is/c++draft/execution.syn):

namespace std {
  // [execpol.type], execution policy type trait
  template<class T> struct is_execution_policy;
  template<class T> constexpr bool is_execution_policy_v = is_execution_policy<T>::value;
}

[akukanov]

OK, seems inline is necessary for C++17/20. Thanks, I will get it back.

[rarutyun]

Suggested change

	constexpr bool is_execution_policy_v = oneapi::dpl::execution::is_execution_policy<T>::value;
	inline constexpr bool is_execution_policy_v = oneapi::dpl::execution::is_execution_policy<T>::value;

[akukanov]

Same here.

[rarutyun]

Suggested change

	to the same class template. It is unspecified in which namespace the underlying class template and its specializations
	are defined.
	to the same class template. It is unspecified, which namespace the underlying class template and its specializations
	are defined in.

[akukanov]

Spellcheckers do not complain :), so I will keep this intact.

[rarutyun]

Suggested change

	For writing new code, use of the type trait utilities in ``namespace oneapi::dpl`` is strongly recommended. Those
	For a new code, use of the type trait utilities in ``namespace oneapi::dpl`` is highly encouraged. Those

[akukanov]

Made a similar change.

[rarutyun]

What's the point of this super generic and kind of vague wording?

I specifically mean "is one of the above specified or implementation-defined oneDPL execution policy types".

[akukanov]

Compare with the C++ standard wording:

is_execution_policy<T> is a Cpp17UnaryTypeTrait with a base characteristic of true_type if T is the type of a standard or implementation-defined execution policy, otherwise false_type.

It is assumed that an implementation can define additional policy types, for which the trait also resolves to true_type.