Update docs by not using is_aggregate (fixes #158) and by describing the get_name implementation (fixes #150)

Author: apolukhin

Diff for: doc/pfr.qbk

@@ -161,7 +161,7 @@ void Write(Writer& writer, int value);
 void Write(Writer& writer, std::string_view value);
 
 template <typename T>
-std::enable_if_t<std::is_aggregate_v<T>> Write(Writer& writer, const T& value) {
+std::enable_if_t<boost::pfr::is_implicitly_reflectable_v<T>> Write(Writer& writer, const T& value) {
   boost::pfr::for_each_field(
       value, [&writer](const auto& field) { Write(writer, field); });
 }
@@ -551,6 +551,8 @@ parameters provided to `BOOST_PFR_CORE_NAME_PARSING` macro [*and] the initial ou
 
 [section How it works]
 
+[h2 Fields count detection and getting references to members]
+
 Short description:
 
 # at compile-time: use aggregate initialization to detect fields count in user-provided structure
@@ -577,6 +579,17 @@ Long description of some basics: [@https://youtu.be/UlNUNxLtBI0 Antony Polukhin:
 Long description of some basics of C++14 with [link boost_pfr.limitations_and_configuration [*BOOST_PFR_USE_LOOPHOLE == 0]]: [@https://youtu.be/abdeAew3gmQ Antony Polukhin: C++14 Reflections Without Macros, Markup nor External Tooling].
 Description of the [*BOOST_PFR_USE_LOOPHOLE == 1] technique by its inventor Alexandr Poltavsky [@http://alexpolt.github.io/type-loophole.html in his blog].
 
+[h2 Field name retrieval]
+
+# at compile-time:
+    * Get references to members of an object of type `T` in `constexpr` function
+    * Feed the reference from previous as a template parameter to a `constexpr` function with `template <auto member_ptr>`.
+      That function returns `__PRETTY_FUNCTION__` or some other vendor specific macro that prints the whole name of a function
+      along with the template arguments.
+    * The returned value from previous step contains the member name ([@https://godbolt.org/z/K4aWdcE9G godbolt example]). Do some
+      compiler specific parsing of the value and make a `std::string_view` that contains only the member name.
+# at run-time: return the `std::string_view` with the member name.
+
 [endsect]
 
 [section Acknowledgements]