Update playbooks2.rst

Added doc on encrypt capability of vars_prompt.
Assuming the example in https://github.com/ansible/ansible/blob/devel/examples/playbooks/prompts.yml is still valid.
Adds crypt scheme list for vars_prompt

Added more details on encrypt usage for vars_prompt, including a list of
crypt schemes and a ling to the passlib documentation.
Cosmetic fixes

Fixed typo in passlib URL link
Fixed style in cryp scheme list
Fixed bullet 7 in variable precedence section
Adds statement regarding passlib requirement
Merge pull request #2302 from leucos/patch-1

Update playbooks2.rst