Settle the documentation of the iterators

We choose to hide the iterator type definitions and just document their
category, which is enough.

doc/mrdocs.yml

@@ -12,6 +12,9 @@ cmake: '-DDYNAMIC_BITSET_MRDOCS_BUILD=ON'
 input:
   - ../include
 exclude-symbols:
+  - 'boost::bit_iterator_base'
+  - 'boost::bit_iterator'
+  - 'boost::const_bit_iterator'
   - 'boost::detail'
   - 'boost::detail::**'
   - 'boost::to_string_helper'

include/boost/dynamic_bitset/dynamic_bitset.hpp

@@ -271,10 +271,19 @@ public:
     friend class const_bit_iterator< dynamic_bitset >;
 
     //!     A read/write iterator into the bitset.
+    //!
+    //!     If `AllocatorOrContainer` is an allocator type, this is a
+    //!     C++20 RandomAccessIterator; otherwise, its category is the
+    //!     corresponding "non-legacy" category of the iterator type of
+    //!     the underlying container; for instance, if the underlying
+    //!     container provides LegacyBidirectionalIterators, this is a
+    //!     BidirectionalIterator.
     // -----------------------------------------------------------------------
     typedef bit_iterator< dynamic_bitset > iterator;
 
     //!     A read-only iterator into the bitset.
+    //!
+    //!     \copydetails iterator
     // -----------------------------------------------------------------------
     typedef const_bit_iterator< dynamic_bitset > const_iterator;