Enhance document.
Enhance document.

Add mention about:
  1) required object privileges
  2) commands which are not available with auto transaction management
  3) log_statement configuration for logging activity of dblink_plus
  4) handling of multi-statement in dblink.call()
  5) possible memory leak caused by aborting a query

bwtakacy/prev_dblink_plus_repo,bwtakacy/prev_dblink_plus_repo