Add conditional noexcept forwarding from free to member swap functions.

This allows std::is_nothrow_swappable{,_with} to work properly.
This only touches user-facing classes, with the following exceptions:
- map,set: only doc changes (with a drive-by missing semicolon fix),
  detail::tree which they derive from already forwards noexcept for swap.
- flat_set: only doc changes for flat_set and a change for
  detail::flat_tree, which it derives from to get swap.
- static_vector: while this adds the forwarding, member static_vector::swap
  is currently never noexcept, although it could be for noexcept-swappable
  and noexcept-move-constructible elements.
- small_vector: there is no free swap, and member swap is never noexcept,
  although it could be for noexcept-swappable and noexcept-move-
  constructible elements. Swapping small_vectors of exact same type goes
  through std::swap, which seems to get noexcept right, and small_vectors
  of different sizes use swap for boost::container::vector, which fails
  to detect when such swap could be noexcept. This patch doesn't touch
  small_vector.
- scoped_allocator: neither member nor free swap are noexcept, although
  they should be, and all other constructions/assignments/comparisons too,
  as they only forward these operations to underlying allocators, which
  are required to not throw exceptions for them. Only static_vector's
  static_storage_allocator might throw for these and it shouldn't appear
  in scoped_allocator. Unsure of the scope of changes needed or whether it
  is even worth it, this patch doesn't touch scoped_allocator.

include/boost/container/flat_set.hpp

@@ -1084,7 +1084,9 @@ class flat_set
    //! <b>Effects</b>: x.swap(y)
    //!
    //! <b>Complexity</b>: Constant.
-   friend void swap(flat_set& x, flat_set& y);
+   friend void swap(flat_set& x, flat_set& y)
+      BOOST_NOEXCEPT_IF(  allocator_traits_type::is_always_equal::value
+                                 && boost::container::dtl::is_nothrow_swappable<Compare>::value );
 
    //! <b>Effects</b>: Extracts the internal sequence container.
    //!
@@ -1817,7 +1819,9 @@ class flat_multiset
    //! <b>Effects</b>: x.swap(y)
    //!
    //! <b>Complexity</b>: Constant.
-   friend void swap(flat_multiset& x, flat_multiset& y);
+   friend void swap(flat_multiset& x, flat_multiset& y)
+      BOOST_NOEXCEPT_IF(  allocator_traits_type::is_always_equal::value
+                                 && boost::container::dtl::is_nothrow_swappable<Compare>::value );
 
    //! <b>Effects</b>: Extracts the internal sequence container.
    //!