Documentation: explain how the BeautifulSoup parser can be specified

[scorchio]

In the following part of the documentation,

MechanicalSoup/mechanicalsoup/stateful_browser.py

Lines 29 to 35 in 16238f4

	:param soup_config: Configuration passed to BeautifulSoup to affect
	the way HTML is parsed. Defaults to ``{'features': 'lxml'}``.
	If overridden, it is highly recommended to `specify a parser
	<https://www.crummy.com/software/BeautifulSoup/bs4/doc/#specifying-the-parser-to-use>`__.
	Otherwise, BeautifulSoup will issue a warning and pick one for
	you, but the parser it chooses may be different on different
	machines.

...it's mentioned that it's recommended to specify a parser, but looking at BeautifulSoup's documentation, the connection between the features and how the parser is specified is not evident because the BS doc never uses the features keyword, rather just tosses it in as the second argument:

(https://beautiful-soup-4.readthedocs.io/en/latest/#differences-between-parsers)

BS4's documentation also doesn't include an API reference that would list the different parameters, it only provides a high-level intro.

I would recommend modifying the MechanicalSoup docs to explain the parser setting more explicitly, like how BS4 does it in code:

:param features: Desirable features of the parser to be
         used. This may be the name of a specific parser ("lxml",
         "lxml-xml", "html.parser", or "html5lib") or it may be the
         type of markup to be used ("html", "html5", "xml"). It's
         recommended that you name a specific parser, so that
         Beautiful Soup gives you the same results across platforms
         and virtual environments.