Document lack of versioning in request method.

[martey]

@dursk raised a great and prescient point in an message on the Google Group several months ago:

Either request() should be marked private as _request(), or, since the API calls funnel through request(), the versioning should be handled within this method rather than passed to this method by all callers.

Since then, multiple people have suggested the latter solution (see #284 and #302). As I mentioned in those issues, I think the latter solution will end up causing additional issues. I think the former solution (suggesting that the request method is private and should not be used unless you know what you are doing) is better.

[martey]

Also see e3a7c11, which removes versioning from request().

[martey]

I think I disagree with my earlier comments. Making the request method private might stop people from using it, which isn't really what we want (request() exists so that advanced Graph API functionality not covered by the builtin methods can still be used).

I think a better solution would be to document the fact that request() doesn't use versions for flexibility, and that people using it need to add it in themselves. I am keeping this on the v3.0.0 milestone since people understandably continue to be confused by this (see #382 for example).