[async.assoc.alloc.members] Improve wording for consistency with IS

Also fixes overfull hbox warnings.

Author: jwakely

src/async.tex

@@ -960,7 +960,9 @@
 
 \begin{itemdescr}
 \pnum
-\ctype If \tcode{T} has a nested type \tcode{allocator_type}, \tcode{typename T::allocator_type}. Otherwise \tcode{ProtoAllocator}.
+\ctype If the \grammarterm{qualified-id} \tcode{T::allocator_type}
+is valid and denotes a type~(\CppXref{temp.deduct}), \tcode{T::allocator_type}.
+Otherwise \tcode{ProtoAllocator}.
 \end{itemdescr}
 
 \begin{itemdecl}
@@ -969,12 +971,13 @@
 
 \begin{itemdescr}
 \pnum
-\returns If \tcode{T} has a nested type \tcode{allocator_type}, \tcode{t.get_allocator()}. Otherwise \tcode{a}.
+\returns If the \grammarterm{qualified-id} \tcode{T::allocator_type}
+is valid and denotes a type~(\CppXref{temp.deduct}), \tcode{t.get_allocator()}.
+Otherwise \tcode{a}.
 \end{itemdescr}
 
 
 
-
 \rSec1[async.assoc.alloc.get]{Function \tcode{get_associated_allocator}}
 
 \indexlibrary{\idxcode{get_associated_allocator}}%