add doc: is_increasing, is_decreasing

source/libs/sprout/algorithm/is_decreasing.rst

@@ -0,0 +1,42 @@
+.. _sprout-algorithm-is_decreasing:
+###############################################################################
+is_decreasing
+###############################################################################
+
+Interface
+========================================
+.. sourcecode:: c++
+
+  template<typename ForwardIterator>
+  inline SPROUT_CONSTEXPR bool
+  is_decreasing(ForwardIterator first, ForwardIterator last);
+
+Returns
+========================================
+
+| Same as ``is_sorted(first, last, greater<>())``.
+
+Examples
+========================================
+.. sourcecode:: c++
+
+  #include <sprout/algorithm/is_decreasing.hpp>
+  #include <sprout/array.hpp>
+  #include <sprout/container.hpp>
+  using namespace sprout;
+
+  SPROUT_STATIC_CONSTEXPR auto input = array<int, 10>{{5, 5, 4, 4, 3, 3, 2, 2, 1, 1}};
+  SPROUT_STATIC_CONSTEXPR auto result = sprout::is_decreasing(begin(input), end(input));
+  static_assert(result, "input is decreasing.");
+
+Complexity
+========================================
+
+| Recursive function invocations in *O(logN)* (logarithmic) depth.
+
+Header
+========================================
+
+| ``sprout/algorithm/is_decreasing.hpp``
+| Convenience header: ``sprout/algorithm.hpp``
+