[docs] [issue10787] [random.gammavariate] Add the expression of the distribution in a comprehensive form for random.gammavariate

[Raymond Hettinger]

Changes by Raymond Hettinger <rhettinger at users.sourceforge.net>:


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assignee: docs at python -> rhettinger

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue10787>
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