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.

Author: jgopel

doc/algorithm.qbk

@@ -111,6 +111,11 @@ Apply a functor to the elements of a sequence
 Transform each element in a range then combine adjacent elements to create an output range. Inclusive scaning means that the nth input is present in the nth output.
 [endsect:transform_inclusive_scan]
 
+[section:transform_exclusive_scan transform_exclusive_scan]
+[*[^[link boost.algorithm.transform_exclusive_scan                   transform_exclusive_scan]    ] ]
+Transform each element in a range then combine adjacent elements to create an output range. Exclusive scanning means that the nth input is not present in the nth output.
+[endsect:transform_exclusive_scan]
+
 [endsect:CXX17_inner_algorithms]
 
 [endsect:CXX17]
@@ -239,7 +244,6 @@ Raise a value to an integral power ([^constexpr] since C++14)
 * [*[^[link header.boost.algorithm.cxx17.exclusive_scan_hpp            exclusive_scan]              ] ]
 * [*[^[link header.boost.algorithm.cxx17.inclusive_scan_hpp            inclusive_scan]              ] ]
 * [*[^[link header.boost.algorithm.cxx17.reduce_hpp                    reduce]                      ] ]
-* [*[^[link header.boost.algorithm.cxx17.transform_exclusive_scan_hpp  transform_exclusive_scan]    ] ]
 * [*[^[link header.boost.algorithm.cxx17.transform_reduce_hpp          transform_reduce]            ] ]
 
 [endsect:not_yet_documented_cxx17_algos]

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,