Clarify documentation for meta argument

[koroldavid]

In the full documentation there are no clear definitions for the meta argument, which is used for all requests.

Also seems different methods require different shapes of meta argument.
For instance, I have used Mixpanel.people_set method passing $latitude and $longitude through meta argument to update account geolocation and it works perfectly. Yet sending those values through meta argument in Mixpanel.track method didn't produce any effect. It occurred I had to pass geolocation through properties parameter in Mixpanel.track

Yet after reading this answer for the related issue #47, I get a false assumption meta will be common for all requests.
Based on this quote:

and the second one (meta) manages metadata about the operation itself.