add environment variables section to manual page

The pre and post executables will not inherit environment.

Reference: 2060a393fe511317d151b09d39e6dd75be753ef6
Signed-off-by: Sami Kerola <sami.kerola@sabre.com>
fix a typo in manual

Signed-off-by: Sami Kerola <sami.kerola@sabre.com>