Mention expand_log in the docstring of logcombine

The documentation of simplify module,
http://docs.sympy.org/dev/modules/simplify/simplify.html
mentions logcombine but not its opposite, expand_log. Often it's
expand_log that one needs to simplify an expression with logarithms.
Added a reference to expand_log to the "See Also" section of logcombine
docstring.

Author: normalhuman

sympy/simplify/simplify.py

@@ -908,6 +908,8 @@ def logcombine(expr, force=False):
     See Also
     ========
     posify: replace all symbols with symbols having positive assumptions
+    sympy.core.function.expand_log: expand the logarithms of products
+        and powers; the opposite of logcombine
 
     """