[async.reqmts.executor] Introduce type Func

Fixes NB GB-5 (PDTS)

Author: jwakely

src/async.tex

@@ -227,9 +227,9 @@
 In Table~\ref{tab:async.reqmts.executor.requirements},
 \tcode{x1} and \tcode{x2} denote values of type \tcode{X} or \tcode{const X},
 \tcode{mx1} denotes an xvalue of type \tcode{X},
-\tcode{f} denotes a \tcode{MoveConstructible} (\CppXref{moveconstructible}) function object callable with zero arguments,
+\tcode{f} denotes a function object of \tcode{MoveConstructible} (\CppXref{moveconstructible}) type \tcode{Func} such that \tcode{f()} is a valid expression,
 \tcode{a} denotes a value of type \tcode{A} or \tcode{const A}
-where \tcode{A} is a type meeting the \tcode{Allocator} requirements (\CppXref{allocator.requirements}),
+where \tcode{A} is a type meeting the \tcode{ProtoAllocator} requirements (\ref{async.reqmts.proto.allocator}),
 and \tcode{u} denotes an identifier.
 
 \begin{libreqtab3}