finished updating docs up to tutorial section

example/features/function_comments.cpp

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+
+#include <limits>
+#include <cassert>
+
+//[no_contracts
+int inc(int& x)
+    // Precondition:    x < std::numeric_limits<int>::max()
+    // Postcondition:   x == oldof(x) + 1
+    //                  result == oldof(x)
+{
+    return x++;
+}
+//]
+
+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;
+}
+