Improve edge API docs.

- create edge operation with 202 result is returned when waitForSync was
set to false
- add missing If-None-Match header parameter description to read head
operation
- add information about _key attribute in result for replace, update and
delete operations
reverted non-fix
removed debug output
Merge pull request #1186 from yojimbo87/devel

Improve edge API docs.
Merge branch 'devel' of github.com:triAGENS/ArangoDB into v8-upgrade
Merge branch 'v8-upgrade' of github.com:triAGENS/ArangoDB into v8-upgrade