Add documentation to transform_exclusive_scan

Problem:
- There is no documentation for the existing functions. This will make
  it harder for users to consume these functions, espcially as new
  variants are added.

Solution:
- Add documentation.

include/boost/algorithm/cxx17/transform_exclusive_scan.hpp

@@ -22,6 +22,21 @@
 
 namespace boost { namespace algorithm {
 
+/// \fn transform_exclusive_scan ( InputIterator first, InputIterator last, OutputIterator result, BinaryOperation bOp, UnaryOperation uOp, T init )
+/// \brief Transforms elements from the input range with uOp and then combines
+/// those transformed elements with bOp such that the n-1th element and the nth
+/// element are combined. Exclusivity means that the nth element is not
+/// included in the nth combination.
+/// \return The updated output iterator
+///
+/// \param first  The start of the input sequence
+/// \param last   The end of the input sequence
+/// \param result The output iterator to write the results into
+/// \param bOp    The operation for combining transformed input elements
+/// \param uOp    The operation for transforming input elements
+/// \param init   The initial value
+///
+/// \note This function is part of the C++17 standard library
 template<class InputIterator, class OutputIterator, class T,
          class BinaryOperation, class UnaryOperation>
 OutputIterator transform_exclusive_scan(InputIterator first, InputIterator last,