C++ named requirements: Formatter (since C++20)

Formatter is a type that abstracts formatting operations for a given formatting argument type and character type. Specializations of std::formatter provided by the standard library are required to meet the requirements of Formatter except as noted otherwise.

# Defect reports

DRApplied toBehavior as publishedCorrect behavior
LWG 3892C++20the value of pc.begin() was unclear if format-spec is not presentmade clear