more explicit section about thread safety with Session

[Mike Bayer]

should be its own section, perhaps a side box, and should be very clear that all objects associated with a Session are themselves extensions of that Session's state. We don't make this clear right now.

[Mike Bayer]

note session rewrites which have occurred in 43b8346b3c14a5742080ff66ce03bb05e5891c50 9a4a565212fcc9db33b925addecfcc8560ba5445 7fa59a431e1a66bfd383fc45cee5b0196c645678

[Mike Bayer]

most of that content is merged to 0.7 in a035c6b6e9656b5a3c2cf0ed1595a1f1540a58ad

[Mike Bayer]

Removing milestone: 0.7.9 (automated comment)