Add additional information to formatting docs

Author: mborland

doc/modules/ROOT/pages/format.adoc

@@ -8,6 +8,8 @@ https://www.boost.org/LICENSE_1_0.txt
 = Formating support
 :idprefix: format_
 
+Boost.Decimal supports formatting with both `<format>` (when C++20 and header are both available) and `<fmt/format.h>`.
+
 [#std_format]
 == `<format>`
 
@@ -69,8 +71,8 @@ int main()
 [#fmt_format]
 == `<fmt/format.h>`
 
-Support for fmtlib is available as long as `<fmt/format.h>` is present.
-All of the above information on modifiers is the same for fmtlib
+Support for `{fmt}` is available as long as `<fmt/format.h>` is present.
+All of the above information on modifiers is the same for fmtlib, just in a different namespace (i.e. `fmt::` instead of `std::`).
 
 [source, c++]
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