Fix doc for Span.

src/common/span.h

@@ -165,7 +165,7 @@ class SpanIterator {
   std::ptrdiff_t index_ = 0;
 };
 
-/*
+/*!
  * The extent E of the span returned by subspan is determined as follows:
  *
  *   - If Count is not std::dynamic_extent, Count;
@@ -180,7 +180,7 @@ constexpr std::ptrdiff_t calculate_extent(std::ptrdiff_t _extent,
       _count : (_extent != dynamic_extent ? _extent - _offset : _extent);
 }
 
-/*
+/*!
  * If N is dynamic_extent, the extent of the returned span E is also
  * dynamic_extent; otherwise it is std::ptrdiff_t(sizeof(T)) * N.
  */
@@ -246,7 +246,7 @@ XGBOOST_DEVICE bool lexicographical_compare(InputIt1 first1, InputIt1 last1,
 }  // namespace detail
 
 
-/*
+/*!
  * \brief span class implementation, based on ISO++20 span<T>. The interface
  *      should be the same.
  *
@@ -437,7 +437,7 @@ class Span {
     return subspan(size() - _count, _count);
   }
 
-  /*
+  /*!
    * If Count is std::dynamic_extent, r.size() == this->size() - Offset;
    * Otherwise r.size() == Count.
    */