twisted.python.log.callWithContext has no docstring, but is part of the public API

New submission from spiv <[email protected]>:

`twisted.python.log.callWithContext` should have a docstring, as it appears to be a part of the public API of the `log` module.  It should probably refer to the documentation for `twisted.python.context`, and give an example like:

{{{
For example, to change the "system" in the log prefix for the 
duration of a call to `myFrob(xyzzy)`, you can do::

  callWithContext({'system': 'My Frobnicator 2.0'}, myFrob, xyzzy)
}}}

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Type     : enhancement
Component: core
Keywords : documentation
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/4263