Update documentation

doc/system/changes.adoc

@@ -11,7 +11,7 @@ https://www.boost.org/LICENSE_1_0.txt
 ## Changes in Boost 1.78
 
 * Added support for source locations to `error_code`.
-* Added `error_code::to_string`, `error_condition::to_string`.
+* Added `error_code::to_string`, `error_condition::to_string`, `error_code::what`.
 * `system_error::what()` now contains the source location, if present.
 * Added `result<T, E = error_code>`, a class holding either a value or an
   error, defined in `<boost/system/result.hpp>`.

doc/system/reference.adoc

@@ -668,6 +668,10 @@ public:
     template<class charT, class traits>
       friend std::basic_ostream<charT, traits>&
         operator<<( basic_ostream<charT, traits>& os, const error_code & ec );
+
+    // what
+
+    std::string what() const;
 };
 
 // non-member functions
@@ -1054,6 +1058,18 @@ template <class charT, class traits>
 Effects: :: `os << to_string()`.
 Returns: :: `os`.
 
+#### what
+
+```
+std::string what() const;
+```
+[none]
+* {blank}
++
+Returns: :: A string representation of `*this`, suitable for logging and
+  diagnostic output. Typically incorporates `message()`, `to_string()`, and
+  `location().to_string()` (if available.)
+
 #### Nonmembers
 
 ```