Add note about autodiff::seed() to the docs (#1597)

sphinx_docs/source/autodiff.rst

@@ -40,6 +40,12 @@ We can then use ``autodiff::val(result)`` or
 x.  The ``static_cast`` version has the advantage of working on normal
 numbers as well.
 
+``autodiff::seed(x)`` sets the derivative term of ``x`` to 1 (it is equivalent
+to ``x.grad = 1.0``), which effectively tells the code that
+:math:`\frac{\partial x}{\partial x} = 1`. This propagates through any
+operations that ``f(x)`` does, and we end up with :math:`\frac{\partial
+f(x)}{\partial x}` in the derivative term of ``result``.
+
 
 Derivatives of multi-variable functions
 =======================================