Update header include information in test and docs

Author: mborland

doc/modules/ROOT/pages/format.adoc

@@ -71,13 +71,15 @@ int main()
 [#fmt_format]
 == `<fmt/format.h>`
 
-Support for `{fmt}` is available as long as `<fmt/format.h>` is present.
+Support for `{fmt}` is also available.
 All the above information on modifiers is the same for fmtlib, just in a different namespace (i.e. `fmt::` instead of `std::`).
+The header `<boost/decimal/fmt_format.hpp>` is *NOT* part of the convenience header, because it is an optional dependency on a potentially compiled library.
 
 [source, c++]
 ----
 #include <fmt/format.h>
-#include <boost/decimal.hpp> // or <boost/decimal/fmt_format.hpp>
+#include <boost/decimal.hpp>
+#include <boost/decimal/fmt_format.hpp>
 #include <iostream>
 
 int main()

test/test_format_fmtlib.cpp

@@ -9,6 +9,7 @@
 
 #define FMT_HEADER_ONLY
 #include <boost/decimal.hpp>
+#include <boost/decimal/fmt_format.hpp>
 #include <boost/core/lightweight_test.hpp>
 #include <limits>