deprecation warning about features='lxml' parser doesn't specify kwargs nor do docstrings

[Hobson Lane]

Second to last line of warning message could be more informative, since the docstring is not.

    ```
    >>> from bs4 import BeautifulSoup
    >>> soup = BeautifulSoup(markup=open('/Users/hobs/Dropbox/Docs/projects/deletefacebook/facebook-hobsonlane/html/contact_info.htm').read())

    /Users/hobsonlane/anaconda3/envs/nlpiaenv/lib/python3.6/site-packages/bs4/__init__.py:181:
    UserWarning: No parser was explicitly specified, so I'm using the best available HTML parser for this system ("lxml"). This usually isn't a problem, but if you run this code on another system, or in a different virtual environment, it may use a different parser and behave differently.

    The code that caused this warning is on line 11 of the file /Users/hobsonlane/anaconda3/envs/nlpiaenv/bin/ipython. To get rid of this warning, change code that looks like this:

     BeautifulSoup(YOUR_MARKUP})

    to this:

     BeautifulSoup(YOUR_MARKUP, "lxml")

      markup_type=markup_type))
    ```

[Leonard Richardson (leonardr)]

In revision 451 I improved the warning message and added a docstring to the constructor.