better documentation of how to use Session

[jreiberkyle]

The Session class implements rate limiting which allows for smooth asynchronous communication with Planet servers. However, this rate-limiting only works with one Session being used. Multiple Sessions will bypass rate-limiting. This is a very important concept for properly using Session but it isn't in our docs and in fact we even forget how to use it properly (see #759, which I totally missed on my PR review of the example).

Let's clearly document (and highlight) proper usage of Session (demonstrated in orders_create_and_download_multiple_orders.py)

[cholmes]

Would it be reasonable to mostly just link from the docs to the example @jreiberkyle ? Or you also want us to spell out a specific section on rate limiting and advising not to use multiple sessions?

[JuLeeAtPlanet]

Draft provided to @jreiberkyle in off-channel comms.

[JuLeeAtPlanet]

This is a duplicate issue in that it is a sub-issue of the review of the user guide Ticket #799 So I'm closing this issue.