The correct way to destroy a session is by using the context.

  context->destroy_session

Calling session->destroy does not work. The session object doesn't know how
to destroy itself.

Closes #368
Fix the name of the Cookie class in the manual.
Additional documentation fix on how to destroy a session.
Merge branch 'doc/manual' into devel

* doc/manual:
  Additional documentation fix on how to destroy a session.
  Fix the name of the Cookie class in the manual.
  The correct way to destroy a session is by using the context.