[iterator.range] Turn list of headers into a table

It is tricky to properly line-break the list of additional headers
that contain the range functions, and that list is growing quite
large to be displayed as a list in a single sentence.  Prefer to
render the list as a table instead.

Retained the cross-references to the header synopses from the
original text, but they look odd in the table format.

Author: AlisdairM

source/iterators.tex

@@ -7253,26 +7253,31 @@
 In addition to being available via inclusion of the \libheader{iterator} header,
 the function templates in \ref{iterator.range} are available when any of the following
 headers are included:
-\libheaderref{array},
-\libheaderref{deque},
-\libheaderrefx{flat_map}{flat.map.syn},
-\libheaderrefx{flat_set}{flat.set.syn},
-\libheaderrefx{forward_list}{forward.list.syn},
-\libheaderref{hive},
-\libheaderrefx{inplace_vector}{inplace.vector.syn},
-\libheaderref{list},
-\libheaderrefx{map}{associative.map.syn},
-\libheaderref{optional},
-\libheaderrefx{regex}{re.syn},
-\libheaderrefx{set}{associative.set.syn},
-\libheaderref{span},
-\libheaderref{stacktrace},
-\libheaderref{string},
-\libheaderrefx{string_view}{string.view.synop},
-\libheaderrefx{unordered_map}{unord.map.syn},
-\libheaderrefx{unordered_set}{unord.set.syn}
-\libheaderref{valarray}, and
-\libheaderref{vector}.
+\begin{multicolfloattable}{Additional range function headers}{range.function.headers}
+{lll}
+\libheaderref{array} \\
+\libheaderref{deque} \\
+\libheaderrefx{flat_map}{flat.map.syn} \\
+\libheaderrefx{flat_set}{flat.set.syn} \\
+\libheaderrefx{forward_list}{forward.list.syn} \\
+\libheaderref{hive} \\
+\libheaderrefx{inplace_vector}{inplace.vector.syn} \\
+\columnbreak
+\libheaderref{list} \\
+\libheaderrefx{map}{associative.map.syn} \\
+\libheaderref{optional} \\
+\libheaderrefx{regex}{re.syn} \\
+\libheaderrefx{set}{associative.set.syn} \\
+\libheaderref{span} \\
+\libheaderref{stacktrace} \\
+\columnbreak
+\libheaderref{string} \\
+\libheaderrefx{string_view}{string.view.synop} \\
+\libheaderrefx{unordered_map}{unord.map.syn} \\
+\libheaderrefx{unordered_set}{unord.set.syn} \\
+\libheaderref{valarray} \\
+\libheaderref{vector} \\
+\end{multicolfloattable}
 
 \indexlibrary{\idxcode{begin(C\&)}}%
 \begin{itemdecl}