MINOR: log: provide log origin in logformat expressions using '%OG'

MINOR: log: provide log origin in logformat expressions using '%OG'

'%OG' logformat alias may be used to report the log origin (when/where)
that triggered log generation using sess_build_logline().

Possible values are:
  - "sess_error": log was generated during session error handling
  - "sess_killed": log was generated during session abortion (killed
    embryonic session)
  - "txn_accept": log was generated right after frontend conn was accepted
  - "txn_request": log was generated after client request was received
  - "txn_connect": log was generated after backend connection establishment
  - "txn_response": log was generated during server response handling
  - "txn_close": log was generated at the final txn step, before closing
  - "unspec": unknown or not specified

Documentation was updated.