[container.opt.reqmts] Index 3-way compare for containers (#6220)

All of the standard containers are 3-way comparable, but the
specification is provided under optional container requirements
so does not show up in the index.

source/containers.tex

@@ -856,6 +856,26 @@
 then the operations described below
 are implemented by constexpr functions.
 
+% Local command to index a name as a member of all containers.
+\renewcommand{\indexcont}[1]{%
+\indexlibrarymisc{\idxcode{#1}}{optional container requirements}%
+\indexlibrarymemberx{array}{#1}%
+\indexlibrarymemberx{deque}{#1}%
+\indexlibrarymemberx{forward_list}{#1}%
+\indexlibrarymemberx{list}{#1}%
+\indexlibrarymemberx{vector}{#1}%
+\indexlibrarymemberx{map}{#1}%
+\indexlibrarymemberx{set}{#1}%
+\indexlibrarymemberx{multiset}{#1}%
+\indexlibrarymemberx{multimap}{#1}%
+\indexlibrarymemberx{flat_map}{#1}%
+\indexlibrarymemberx{flat_set}{#1}%
+\indexlibrarymemberx{flat_multiset}{#1}%
+\indexlibrarymemberx{flat_multimap}{#1}%
+\indexlibrarymemberx{basic_string}{#1}%
+}
+
+\indexcont{operator<=>}%
 \begin{itemdecl}
 a <=> b
 \end{itemdecl}