Add to docs

Author: mborland

doc/decimal/charconv.adoc

@@ -67,3 +67,44 @@ NOTE: `BOOST_DECIMAL_CONSTEXPR` is defined if:
  - `\\__GNUC__` >= 9
  - Compiler has: `__builtin_is_constant_evaluated()`
  - C++20 support with: `std::is_constant_evaluated()`
+
+The library offers an additional feature for sizing buffers without specified precision and in general format
+[source, c++]
+----
+namespace boost {
+namespace decimal {
+
+template <typename T>
+struct limits
+{
+    static constexpr int max_chars;
+}
+
+} //namespace decimal
+} //namespace boost
+----
+
+The member can then be used to size buffers such as:
+
+[source, c++]
+----
+#include <boost/decimal.hpp>
+#include <iostream>
+
+int main()
+{
+    using namespace boost::decimal;
+
+    decimal32 val {5, -1};
+
+    char buffer[limits<T>::max_chars];
+
+    auto r_to = to_chars(buffer, buffer + sizeof(buffer), val);
+    *r_to.ptr = '\0';
+
+    std::cout << buffer << std::endl;
+
+    return 0;
+}
+
+----