finished updating docs up to tutorial section

example/features/function.cpp

@@ -0,0 +1,31 @@
+
+#include <limits>
+#include <cassert>
+
+//[function
+#include <boost/contract.hpp>
+
+int inc(int& x) {
+    int result;
+    boost::contract::old_ptr<int> old_x = BOOST_CONTRACT_OLDOF(x);
+    boost::contract::guard c = boost::contract::function()
+        .precondition([&] {
+            BOOST_CONTRACT_ASSERT(x < std::numeric_limits<int>::max());
+        })
+        .postcondition([&] {
+            BOOST_CONTRACT_ASSERT(x == *old_x + 1);
+            BOOST_CONTRACT_ASSERT(result == *old_x);
+        })
+    ;
+
+    return result = x++; // Function body.
+}
+//]
+
+int main() {
+    int x = std::numeric_limits<int>::max() - 1;
+    assert(inc(x) == std::numeric_limits<int>::max() - 1);
+    assert(x == std::numeric_limits<int>::max());
+    return 0;
+}
+