Add note about charconv behavior

Author: mborland

doc/decimal/charconv.adoc

@@ -12,6 +12,11 @@ https://www.boost.org/LICENSE_1_0.txt
 
 The following functions analogous to those from https://en.cppreference.com/w/cpp/header/charconv[<charconv>] are provided:
 
+IMPORTANT: `std::from_chars` has an open issue with LWG here: https://cplusplus.github.io/LWG/lwg-active.html#3081.
+The standard for <charconv> does not distinguish between underflow and overflow like strtod does.
+`boost::decimal::from_chars` modifies `value` in order to communicate this to the user in a divergence from the standard.
+This behavior is the same as that of https://www.boost.org/doc/libs/master/libs/charconv/doc/html/charconv.html#from_chars_usage_notes_for_from_chars_for_floating_point_types[`boost::charconv::from_chars_erange`].
+
 [source, c++]
 ----
 namespace boost {