[allocator.requirements] Fix pointer_to expression and reinstate descriptive variable 'r' (#1656)

Author: jwakely

source/lib-intro.tex

@@ -1872,6 +1872,8 @@
 by calling \tcode{a1.allocate}, where \tcode{a1 == a}   \\ \rowsep
 \tcode{q}       &   a value of type \tcode{XX::const_pointer}
 obtained by conversion from a value \tcode{p}.          \\ \rowsep
+\tcode{r}       &   a value of type \tcode{T\&}
+obtained by the expression \tcode{*p}.                  \\ \rowsep
 \tcode{w}       &   a value of type \tcode{XX::void_pointer} obtained by
   conversion from a value \tcode{p}  \\ \rowsep
 \tcode{x}       &   a value of type \tcode{XX::const_void_pointer} obtained by
@@ -1960,7 +1962,7 @@
 
 \tcode{pointer_traits<\brk{}X::pointer\brk{}>::pointer_to(r)}  &
   \tcode{X::pointer}        &
-                            & \\ \rowsep
+  same as \tcode{p}         & \\ \rowsep
 
 \tcode{a.allocate(n)}   &   \tcode{X::pointer}  &
 Memory is allocated for \tcode{n} objects of type \tcode{T} but objects