X3.Docs: Remove parse_rule implementation details

It is hard to not forget to keep in sync these while anyone interested could simply go to the macro definition, moreover users should not rely on the implementation details and telling this info may make it seem like `parse_rule` is a public API.

X3.Test: Make pow call unqualified

X3.Test: Make pow call unqualified

1.74 changelog

X3: Remove unused header unrefcv.hpp

Changelog for 1.73

X3.Tests: Disable constexpr `with` from rvalue test for Clang 6 and b…

…elow

Changelog update. Closes boostorg#555

X3.Tests: rule_separate_tu: Fix static initialization order fiasco

Test for boostorg#511, removing static_assert