Add info from the docstring for random.gammavariate() to the docs.

Doc/library/random.rst

@@ -163,6 +163,7 @@ be found in any statistics text.
    The end-point value ``b`` may or may not be included in the range
    depending on floating-point rounding in the equation ``a + (b-a) * random()``.
 
+
 .. function:: triangular(low, high, mode)
 
    Return a random floating point number *N* such that ``low <= N <= high`` and
@@ -191,6 +192,12 @@ be found in any statistics text.
    Gamma distribution.  (*Not* the gamma function!)  Conditions on the
    parameters are ``alpha > 0`` and ``beta > 0``.
 
+   The probability distribution function is::
+
+                 x ** (alpha - 1) * math.exp(-x / beta)
+       pdf(x) =  --------------------------------------
+                   math.gamma(alpha) * beta ** alpha
+
 
 .. function:: gauss(mu, sigma)