BUG: MPLPlot cannot make loglog keyword worked
Changes made in #5638 make `loglog` keyword not worked, and it fixes the problem.

looks good to me - thanks for submitting this! Doesn't need a doc note b/c both are within 0.14
 @sinhrks could you add a note to the docstrings that marks the existence of `loglog`? e.g. around line 1881 in `pandas/tool/plotting.py` after the comment descibing `logy`. That would help other users actually know about this.
 Thank you for your confirmation. Docstring added, and I removed descriptions specifying line plot because now log-related keyword can be used in all plots thanks to #5638.
 @sinhrks thanks!
