[predef.iterators] Singularize heading of singular sentinels.

Also adjust stable labels.

Author: jensmaurer

source/iterators.tex

@@ -377,7 +377,7 @@
   template<@\libconcept{input_iterator}@ I, class S>
     struct iterator_traits<common_iterator<I, S>>;
 
-  // \ref{default.sentinels}, default sentinels
+  // \ref{default.sentinel}, default sentinel
   struct default_sentinel_t;
   inline constexpr default_sentinel_t @\libglobal{default_sentinel}@{};
 
@@ -390,7 +390,7 @@
   template<@\libconcept{input_iterator}@ I>
     struct iterator_traits<counted_iterator<I>>;
 
-  // \ref{unreachable.sentinels}, unreachable sentinels
+  // \ref{unreachable.sentinel}, unreachable sentinel
   struct unreachable_sentinel_t;
   inline constexpr unreachable_sentinel_t @\libglobal{unreachable_sentinel}@{};
 
@@ -5091,7 +5091,7 @@
 Equivalent to \tcode{ranges::iter_swap(get<I>(x.v_), get<I2>(y.v_))}.
 \end{itemdescr}
 
-\rSec2[default.sentinels]{Default sentinels}
+\rSec2[default.sentinel]{Default sentinel}
 
 \indexlibraryglobal{default_sentinel_t}%
 \begin{itemdecl}
@@ -5648,9 +5648,7 @@
 Equivalent to \tcode{ranges::iter_swap(x.current, y.current)}.
 \end{itemdescr}
 
-\rSec2[unreachable.sentinels]{Unreachable sentinel}
-
-\rSec3[unreachable.sentinel]{Class \tcode{unreachable_sentinel_t}}
+\rSec2[unreachable.sentinel]{Unreachable sentinel}
 
 \indexlibraryglobal{unreachable_sentinel_t}%
 \pnum