Poison BOOST_DESCRIBE_ENUM in class scope

[ecatmur]

using namespace keyword (is there anything better that can appear at namespace scope, not at class scope)? try to inform the user that they should use BOOST_DESCRIBE_NESTED_ENUM instead

Motivation: we found that people were using BOOST_DESCRIBE_ENUM in nested context and getting confused when it silently failed. This is particularly problematic because nested enums tend to not be enum class so things like std::ostream<< will still work, so we were getting unexpected behavior at runtime (our serialization library prefers Boost.Describe but will fall back to iostreams).

[ecatmur]

test output:

gcc.compile.c++ bin.v2/libs/describe/test/enum_nested_fail.test/gcc-11/debug/cxxstd-14-iso/threading-multi/visibility-hidden/enum_nested_fail.o
In file included from libs/describe/test/enum_nested_fail.cpp:5:
./boost/describe/enum.hpp:74:5: error: expected unqualified-id before ‘namespace’
   74 |     namespace should_use_BOOST_DESCRIBE_NESTED_ENUM {} \
      |     ^~~~~~~~~
libs/describe/test/enum_nested_fail.cpp:15:5: note: in expansion of macro ‘BOOST_DESCRIBE_ENUM’
   15 |     BOOST_DESCRIBE_ENUM(E1);
      |     ^~~~~~~~~~~~~~~~~~~
(failed-as-expected) bin.v2/libs/describe/test/enum_nested_fail.test/gcc-11/debug/cxxstd-14-iso/threading-multi/visibility-hidden/enum_nested_fail.o

and

gcc.compile.c++ bin.v2/libs/describe/test/nested_enum_fail.test/gcc-11/debug/cxxstd-14-iso/threading-multi/visibility-hidden/nested_enum_fail.o
In file included from libs/describe/test/nested_enum_fail.cpp:5:
./boost/describe/enum.hpp:82:33: error: ‘friend’ used outside of class
   82 |     BOOST_DESCRIBE_MAYBE_UNUSED friend BOOST_DESCRIBE_ENUM_BEGIN(E) \
      |                                 ^~~~~~
libs/describe/test/nested_enum_fail.cpp:14:1: note: in expansion of macro ‘BOOST_DESCRIBE_NESTED_ENUM’
   14 | BOOST_DESCRIBE_NESTED_ENUM(E1);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~
(failed-as-expected) bin.v2/libs/describe/test/nested_enum_fail.test/gcc-11/debug/cxxstd-14-iso/threading-multi/visibility-hidden/nested_enum_fail.o