Improve documentation for describe_instances Reservations items

[GabrielHare]

Searching for documentation turns up "reserved instances" which seems to be something different.

The meaning of this field is minimally documented (quoted in entirety):

ReservationId (string) --
The ID of the reservation.

Some developers (my self for one) are working from a "best guess"
https://stackoverflow.com/questions/15618825/what-is-the-purpose-of-reservations-in-amazon-ec2
https://serverfault.com/questions/749118/aws-ec2-what-is-a-reservation-id-exactly-and-what-does-it-represent

OBSERVATION: Working with the EC2 console, the response I receive indicates that all manually launched instances share one reservation, even though they were launched at different times, use different hardware, etc.

Minimum viable fix: provide an explanation here so that it can be cited in the StackOverflow discussion.